Ensure that AIO extra indexes config is well formed

The AIO bootstrap may produce a user_variables.yml config line
which looks like:

repo_build_pip_extra_indexes: [u'http://mirror.dfw.rax.openstack.org/wheel/ubuntu-14.04-x86_64/']

This is not valid for Ansible's consumption.

This patch ensures that the variable content is appropriately
converted to a yaml formatted list which Ansible can consume.

Change-Id: Iebb41f849f112c3a71f4265d366cf1783745664d

tests/roles/bootstrap-host/templates/user_variables.aio.yml.j2

@@ -96,7 +96,8 @@ apply_security_hardening: true
 
 {% if repo_build_pip_extra_indexes is defined %}
 ## Wheel mirrors for the repo_build to use
-repo_build_pip_extra_indexes: {{ repo_build_pip_extra_indexes }}
+repo_build_pip_extra_indexes:
+{{ repo_build_pip_extra_indexes | to_nice_yaml }}
 {% endif %}
 
 {% if nova_uca_enable is defined %}