Zuulv3: drop variable interpolation and add nodesets

The only example of interpolated variables in the spec was in order
to easily specify what kind of node should be used in a single-node
job.  Actually implementing the interpolation is not trivial, and
this kind of interpolation adds some risk around making the
configuration language more complicated, difficult to follow, and
error-prone for users.

Monty suggested that perhaps nodesets could become a first-class
configuration option and negate the need for this particular example.
That seems to work quite well -- allowing sets of nodes to be defined
and referred to by name.

Without this example to stand on, the variable interpolation feature
doesn't seem worth it, at least at the moment.  This change removes
it and adds Nodesets as a configuration item.

If we need it later, we can add it back.  If we need a way to pass
job variables to ansible, we can add a 'vars' job attribute, but treat
it as a simple static structure, without interpolation.

Change-Id: If2de49a698948279fb8deb88eb82bb4ae163f6b3

specs/zuulv3.rst

@@ -183,6 +183,38 @@ file specified that it is permitted::
           repos:
           - stackforge/random  # Specific project config is in-repo
 
+Nodesets
+~~~~~~~~
+
+A significant focus of Zuul v3 is a close interaction with Nodepool to
+both make running multi-node jobs simpler, as well as facilitate
+running jobs on static resources.  To that end, the node configuration
+for a job is introduced as a first-class resource.  This allows both
+simple and complex node configurations to be independently defined and
+then referenced by name in jobs::
+
+  ### global_config.yaml
+  - nodeset:
+      name: precise
+      nodes:
+        - name: controller
+          image: ubuntu-precise
+  - nodeset:
+      name: trusty
+      nodes:
+        - name: controller
+         image: ubuntu-trusty
+  - nodeset:
+      name: multinode
+      nodes:
+        - name: controller
+          image: ubuntu-xenial
+        - name: compute
+         image: ubuntu-xenial
+
+Jobs may either specify their own node configuration in-line, or refer
+to a previously defined nodeset by name.
+
 Jobs
 ~~~~
 
@@ -200,10 +232,7 @@ handled by the Jenkins (or other worker) definition::
   - job:
       name: base
       timeout: 30m
-      node: precise   # Just a variable for later use
-      nodes:  # The operative list of nodes
-        - name: controller
-          image: {node}  # Substitute the variable
+      nodes: precise
       auth:  # Auth may only be defined in central config, not in-repo
         inherit: true  # Child jobs may inherit these credentials
         swift:
@@ -228,7 +257,7 @@ Further jobs may extend and override the remaining parameters::
   - job:
       name: python27
       parent: base
-      node: trusty
+      nodes: trusty
 
 Our use of job names specific to projects is a holdover from when we
 wanted long-lived slaves on Jenkins to efficiently re-use workspaces.
@@ -241,19 +270,21 @@ can add that information back in when reporting statistics.  Jobs may
 have multiple aspects to accomodate differences among branches, etc.::
 
   ### global_config.yaml (continued)
-  # Version that is run for changes on stable/icehouse
+  # Version that is run for changes on stable/diablo
   - job:
       name: python27
       parent: base
-      branches: stable/icehouse
-      node: precise
+      branches: stable/diablo
+      nodes:
+        - name: controller
+          image: ubuntu-lucid
 
   # Version that is run for changes on stable/juno
   - job:
       name: python27
       parent: base
       branches: stable/juno  # Could be combined into previous with regex
-      node: precise          # if concept of "best match" is defined
+      nodes: precise         # if concept of "best match" is defined
 
 Jobs may specify that they require more than one node::
 
@@ -261,12 +292,7 @@ Jobs may specify that they require more than one node::
   - job:
       name: devstack-multinode
       parent: base
-      node: trusty  # could do same branch mapping as above
-      nodes:
-        - name: controller
-          image: {node}
-        - name: compute
-          image: {node}
+      nodes: multinode
 
 Jobs defined centrally (i.e., not in-repo) may specify auth info::
 
@@ -300,7 +326,7 @@ non-voting for a given project in a given pipeline::
         jobs:
           - python27  # Runs version of job appropriate to branch
           - pep8:
-              node: trusty  # override the node type for this project
+              nodes: trusty  # override the node type for this project
           - devstack
           - devstack-deprecated-feature:
               branches: stable/juno  # Only run on stable/juno changes
@@ -328,7 +354,7 @@ to use in-repo configuration::
   - job:
       name: random-job
       parent: base      # From global config; gets us logs
-      node: precise
+      nodes: precise
 
   - project:
       name: stackforge/random