Make auth backend use OPENSTACK_ENDPOINT_TYPE parameter from settings.

And if parameter is missing in dashboard's settings, use default value of
'publicURL'. This default value doesn't fit documentation at
http://docs.openstack.org/developer/horizon/topics/settings.html#openstack-endpoint-type,
but as lin-hua.cheng@hp.com said (see
https://review.openstack.org/#/c/45679/) documentation should
be fixed itself.

More tests were added: same checks for V2 and V3 are also run for
three possible values of settings.OPENSTACK_ENDPOINT_TYPE ('publicURL',
'internalURL' and 'adminURL'). Old test suites correspond to
settings.OPENSTACK_ENDPOINT_TYPE parameter not being set.

Closes-bug: #1221563
Change-Id: Ic164d1cfcb0c7606c7ca51b869a738b2976bf6ea
Merge "Make auth backend use OPENSTACK_ENDPOINT_TYPE parameter from settings."