Added support for authentication against Active Directory subdomains.

This patch enables authentication against specific AD subdomain extracted from
the username. E.g. for configured domain name "example.com" user "north\tom"
will be authenticated as "tom" against "north.example.com".

Testing done:
I have this running fine with multiple AD subdomains configured in my
organization.

Reviewed at https://reviews.reviewboard.org/r/2838/
Update the test suite names for ReviewRequestManager tests.

DbQueryTests had tests named after the old DB query functions we had way
back before introducing ReviewRequestManager. The names and docstrings
were old and outdated. This updates them for the appropriate names,
making it easier to see if they fail, and to introduce new tests.
Merge branch 'release-1.6.x' into release-1.7.x