Write the local.conf file for use by devstack

**Role Variables**

.. zuul:rolevar:: devstack_base_dir
   :default: /opt/stack

   The devstack base directory.

.. zuul:rolevar:: devstack_local_conf_path
   :default: {{ devstack_base_dir }}/devstack/local.conf

   The path of the local.conf file.

.. zuul:rolevar:: devstack_localrc
   :type: dict

   A dictionary of variables that should be written to the localrc
   section of local.conf.  The values (which are strings) may contain
   bash shell variables, and will be ordered so that variables used by
   later entries appear first.

.. zuul:rolevar:: devstack_local_conf
   :type: dict

   A complex argument consisting of nested dictionaries which combine
   to form the meta-sections of the local_conf file.  The top level is
   a dictionary of phases, followed by dictionaries of filenames, then
   sections, which finally contain key-value pairs for the INI file
   entries in those sections.

   The keys in this dictionary are the devstack phases.

   .. zuul:rolevar:: [phase]
      :type: dict

      The keys in this dictionary are the filenames for this phase.

      .. zuul:rolevar:: [filename]
         :type: dict

         The keys in this dictionary are the INI sections in this file.

         .. zuul:rolevar:: [section]
            :type: dict

            This is a dictionary of key-value pairs which comprise
            this section of the INI file.

.. zuul:rolevar:: devstack_services
   :type: dict

   A dictionary mapping service names to boolean values.  If the
   boolean value is ``false``, a ``disable_service`` line will be
   emitted for the service name.  If it is ``true``, then
   ``enable_service`` will be emitted.  All other values are ignored.

.. zuul:rolevar:: devstack_plugins
   :type: dict

   A dictionary mapping a plugin name to a git repo location.  If the
   location is a non-empty string, then an ``enable_plugin`` line will
   be emmitted for the plugin name.