charms.reactive deprecates the RelationBase.from_state() class
method. This is an internal method in charms.reactive and shouldn't
really have been used.
This patch changes to charms.relations.endpoint_from_flag(...) which
essentially does the same thing and will be a more stable API moving
forwards.
Note that from this point on, the library will ONLY work with
charms.reactive 0.6.0 onwards and the modified charms.openstack library.
Change-Id: I6e025438bacd17dfb2a6f61fb8ca274187009c57
b902df7e0e