Imports to fix build warnings

There were warnings generated during the build due to missing
attributes, like this:

 keystoneclient.rst:7: WARNING: missing attribute mentioned in
   :members: or __all__: module keystoneclient.__init__, attribute
   v2_0

This is fixed by importing the symbol in __init__.

Closes-Bug: #1315523

Change-Id: I565c78ef8910353c4e1ce1641b42fba7cdbd77d8
Doc build fails if warnings

If the docstrings are invalid then the commit should fail the gate.

Change-Id: Iabe506fa914ac5e742c0c5f4682ed0471c4a64ca
Unversioned endpoints in service catalog

If you pass a version number to the endpoint_filter then an identity
plugin will make a request to the URL in the service catalog and find an
appropriate URL for the requested version.

It caches the response to each of the discovery queries so that it
should only query once per URL.

This will only work for applications that create session objects
directly as the legacy model does not use the get_endpoint features of
an identity plugin.

This change showed an inconsistency in the docstrings between discovery
and the usage of discovery so the docstring was fixed.

Blueprint: endpoint-version-query
Change-Id: I277f2f6ad6c8cd44f1a9c06cf07d62bc8f8b383b
Merge "Imports to fix build warnings"
Merge "Doc build fails if warnings"
Merge "Unversioned endpoints in service catalog"