This commit contains the interface code to connect between the
manila charm and a backend plugin that will provide configuration
for the manila.
Plugin charms are subordinate because they need to be on the same
unit/machine as the manila file share service.
This interface also implements a 1-many interface and so contains code
to manage that. That code will be abstracted back to charms.openstack
at some point.
Change-Id: Ied0ad014ab7b1d4778113b0d3f2bbae08075372e
Zuul