Update kvs assignment backend docs

The "key to data" mapping was not documented for the KVS
assignment backend.

Change-Id: Id3cf3e0d9b52ceae5099e6d3833cc88f7aa0f347
Do not use auth_info objects for accessing the API

Currently all the plugins in keystone.auth.plugins.external use the
auth_info object for acessing the identity and assignment APIs. This
change removes this and uses keystone.common.dependency instead.

Change-Id: I56f06a9c621b5127ae1ca786631c80207bb4286d
Merge "Do not use auth_info objects for accessing the API"
Merge "Update kvs assignment backend docs"