Context selection for project and region

Providing the mechanism to easily switch between projects and regions.

Only shows domain info if OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT is set
to True in settings.

Will add domain switching for v3 once support for it is complete.
Will add advance filtering rules in follow up patch.

Co-Authored-By: David Lyle <david.lyle@hp.com>

Implements: blueprint context-selection

Change-Id: Ie6e81eebe822ab2394b07345d148a1c06a33a25f
Merge "Context selection for project and region"
Merge branch 'master' of https://github.com/openstack/horizon