Add support for user_id based authentication with Neutron

Keystone v3 supports non-unique project/user names, so until
the Neutron client provides support for the V3 API and domain
based authentication Nova should allow user ID for admin
authentication.

Change-Id: Idad27ff4206d6ba0bb9aedbbff5172c3144ad3c6
DocImpact: Adds new flag, neutron.admin_user_id
Merge "Add support for user_id based authentication with Neutron"