Adding domain to context

Add support for upcoming "domain" concept in Keystone V3 API in
the context.

This fix ensures there is a user_identity in the context for the
corresponding new attribute in the log format string added in oslo. The
oslo change has the same change-id as this one.

Closes-Bug: #1290503
Change-Id: Ic2cf3e52cfcc0b8adccdf9c59afaa4014708a303
Merge "Adding domain to context"