Makes client always use provided endpoint_url

When nova uses neutron-client and the client needs to authenticate with
keystone it will additionally retrieve the service-catalog. The client will
then ignore the provided endpoint_url parameter given to it during
instantiation because the service-catalog retrieval process overrides that
parameter.

Using the endpoint declared in the service-catalog even though the endpoint is
given is unexpected given the precedent set by glance-client to respect that
parameter. Ensuring the parameter is used will allow users to override the
identity's catalog. This is useful if identity is in error, or if the user is
using a special development configuration.

Unit tests are included to test that said functionality occurs.

Fixes: bug #1212812
Change-Id: Iad2eaf4096fd5278740c3664cc369853814ee2d5
Merge "Makes client always use provided endpoint_url"