Add new tripleo::profile::base::glance::api::multistore_config parameter
to support configuring multiple glance-api backends. The parameter is
optional, and represents a hash of settings for each additional backend.
The existing 'glance_backend' parameter specifies the default backend.
In order to support DCN/Edge deployments, the syntax supports multiple
instances of the 'rbd' backend type. Restrictions are imposed to allow
only a single instance of the 'cinder', 'file' and 'swift' backend types.
Change-Id: I41ab9b3593bf3d078c5bbd1826df8308e3f5e7af
Depends-On: I5a1c61430879a910e7b6c79effba538431959d56