Adds unitest2 and keyring to pip-requires

When running tempest outside of a devstack
machine, the unittest2 and keyring packages
are missing from the virtual environment.

Adding the packages to tools/pip-requires
triggers their installation both when
using run_tests.sh as well as tox.

Fixes: LP bug #1112542
Change-Id: I5c3632394a2b0fe364022854d8ff62ef4866ace9