Auth: Allows an auth_url path to contain version

[gildub]

The prefix_path allows to ignore version ('/v3' or '/v2.0') when the identity endpoint (auth_url) contains a version in its path.

[theopenlab-ci[bot]]

Build succeeded.

• fog-openstack-unittest-test : SUCCESS in 13m 37s
• fog-openstack-unittest-spec : SUCCESS in 14m 00s

[theopenlab-ci[bot]]

Build succeeded.

• fog-openstack-unittest-test : SUCCESS in 13m 15s
• fog-openstack-unittest-spec : SUCCESS in 13m 19s

[aufi]

Looks good to me, merging :+1: