Allow plugins to express dependency info

Add a no-op function, "plugin_requires" to allow plugins to indicate
their dependencies on each other.  This will be used by the Devstack
Ansible module when writing local.conf files.

Also add define_plugin to allow plugins to indicate their canonical
names.

Change-Id: Ibd8c7222ed7dfb08d7ea821d871fc6f3b88de24b

doc/source/plugins.rst

@@ -54,6 +54,31 @@ directory. Inside this directory there can be 3 files.
   default value only if the variable is unset or empty; e.g. in bash
   syntax ``FOO=${FOO:-default}``.
 
+  The file should include a ``define_plugin`` line to indicate the
+  plugin's name, which is the name that should be used by users on
+  "enable_plugin" lines.  It should generally be the last component of
+  the git repo path (e.g., if the plugin's repo is
+  openstack/devstack-foo, then the name here should be "foo") ::
+
+    define_plugin <YOUR PLUGIN>
+
+  If your plugin depends on another plugin, indicate it in this file
+  with one or more lines like the following::
+
+    plugin_requires <YOUR PLUGIN> <OTHER PLUGIN>
+
+  For a complete example, if the plugin "foo" depends on "bar", the
+  ``settings`` file should include::
+
+    define_plugin foo
+    plugin_requires foo bar
+
+  Devstack does not currently use this dependency information, so it's
+  important that users continue to add enable_plugin lines in the
+  correct order in ``local.conf``, however adding this information
+  allows other tools to consider dependency information when
+  automatically generating ``local.conf`` files.
+
 - ``plugin.sh`` - the actual plugin. It is executed by devstack at
   well defined points during a ``stack.sh`` run. The plugin.sh
   internal structure is discussed below.

functions-common

@@ -1703,6 +1703,35 @@ function run_phase {
     fi
 }
 
+# define_plugin <name>
+#
+# This function is a no-op.  It allows a plugin to define its name So
+# that other plugins may reference it by name.  It should generally be
+# the last component of the canonical git repo name.  E.g.,
+# openstack/devstack-foo should use "devstack-foo" as the name here.
+#
+# This function is currently a noop, but the value may still be used
+# by external tools (as in plugin_requires) and may be used by
+# devstack in the future.
+#
+# ``name`` is an arbitrary name - (aka: glusterfs, nova-docker, zaqar)
+function define_plugin {
+    :
+}
+
+# plugin_requires <name> <other>
+#
+# This function is a no-op.  It is currently used by external tools
+# (such as the devstack module for Ansible) to automatically generate
+# local.conf files.  It is not currently used by devstack itself to
+# resolve dependencies.
+#
+# ``name`` is an arbitrary name - (aka: glusterfs, nova-docker, zaqar)
+# ``other`` is the name of another plugin
+function plugin_requires {
+    :
+}
+
 
 # Service Functions
 # =================