Exclude ansible_python_interpreter from write-inventory

Zuul now includes an ansible_python_interpreter hostvar in every
host in its inventory.  It defaults to python2.  The write-inventory
role, which takes the Zuul inventory and makes an inventory for
the fake bridge server in the gate passes that through.  Because it's
in /etc/ansible/inventory.yaml, it overrides any settings which may
arrive via group vars, but this is the way we set the interpreter
for all the hosts on bridge (we do not do so in the actual inventory
file).

To correct this, tell write-inventory to strip the
ansible_python_interpreter variable when it writes out the new
inventory.  This restores the behavior to match what happens on
the real bridge host.  One instance of setting the interpreter
for the fake "trusty" host used in base platform tests is moved to
a hostvars file to match the rest of the real hosts.

Change-Id: I60f0acb64e7b90ed8af266f21f2114fd598f4a3c

.zuul.yaml

@@ -379,9 +379,6 @@
           label: ubuntu-xenial
         - name: bionic
           label: ubuntu-bionic
-    host-vars:
-      trusty:
-        ansible_python_interpreter: python2
     files:
       - playbooks/.*
       - roles/.*

playbooks/host_vars/trusty.yaml

@@ -0,0 +1,3 @@
+# This is a fake host used in gate testing of the base playbook on
+# each supported platform.
+ansible_python_interpreter: python2

playbooks/zuul/run-base.yaml

@@ -13,6 +13,7 @@
         write_inventory_dest: /etc/ansible/hosts/inventory.yaml
         write_inventory_exclude_hostvars:
           - ansible_user
+          - ansible_python_interpreter
     - name: Set up /opt/system-config repo
       git:
         repo: /home/zuul/src/opendev.org/opendev/system-config