Expose serial keyword to base-test playbook

Now that zuul supports a single shared ansible_host in inventories,
add the ability for a user to use the serial keyword on our base
playbooks.

When using multi-host inventory files, all of the tasks happen
independent of each other on separate hosts. With a share ansibe_host,
they will end up fighting with each other. One way to avoid this, is
to set the serial keyword to 1, and host will run tasks one at a time.

By default, if base_serial is not set, we'll omit the serial keywork
in the playbooks.

Change-Id: I532939c843e7f6f6a7d29beb84ef523d25536bb3
Depends-On: I674d6baac26852ee1503feb1ed16c279bf773688
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

playbooks/base-test/post-ssh.yaml

@@ -2,5 +2,6 @@
   # NOTE(pabelanger): We ignore_errors for the following tasks as not to fail
   # successful jobs.
   ignore_errors: yes
+  serial: "{{ base_serial | default(omit) }}"
   roles:
     - remove-build-sshkey

playbooks/base-test/pre.yaml

@@ -4,6 +4,7 @@
       zuul_log_url: "http://logs.openstack.org"
 
 - hosts: all
+  serial: "{{ base_serial | default(omit) }}"
   roles:
     - add-build-sshkey
     - start-zuul-console

zuul.d/jobs.yaml

@@ -37,6 +37,18 @@
 
       It also sets default timeout and nodeset values (which may be
       overidden).
+
+      Responds to these variables:
+
+      .. zuul:jobvar:: base_serial
+         :default: Omitted
+
+         This sets the serial keyword in the pre and post playbooks
+         which can be an integer or percentage.
+
+         See ansible documentation for more information:
+         http://docs.ansible.com/ansible/latest/playbooks_delegation.html
+
     pre-run: playbooks/base/pre.yaml
     post-run:
       - playbooks/base/post-ssh.yaml