Add function is_plugin_enabled
Add function is_plugin_enabled

Add a function which tests if a plugin has been enabled with
enable_plugin. This is helpful if two co-ordinating projects want to run
specific setup in devstack in one only if the other is enabled.

Change-Id: Ibf113755595b19d028374cdc1c86e19b5170be4f

dtroyer/devstack,group-policy/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,nawawi/openstack,dtroyer/devstack,group-policy/devstack,noironetworks/devstack,openstack-dev/devstack