Allow user ID for authentication

In Keystone V3 user names are no longer necessarily unique
across domains.

A user can still authenticate a user in the non default
domain via the V2 API providng they use IDs instead of names.

Tenant_ID is already supported, this change adds support
for user ID

Closes-Bug: #1299807

Change-Id: I22fdd9a6749f7dfbdd2dc8313fddc81e5ea0b753
Merge "Allow user ID for authentication"