Keystone backend preparation for domain-scoping

These changes lay the ground work for the implmentation of
domain-scoping, but are benign in that they don't change the token.
They include making domain_id a first-class attribute in the user
and project entity (i.e. move it out of the 'extra' attribute),
filling in domain grant and project support for the kvs backend and
fixing a series of issues in the mirgation to make it work for both
MySQL, Postgresql and sqlite.

A further, separate, commit will actually provide the code to
update the actual tokens once the v3 token support has been added.

blueprint domain-scoping
blueprint default-domain

Change-Id: I55ab7947a6a1efbab003bd234856bd3805bb4a63
Merge "Keystone backend preparation for domain-scoping"