install test-reqs when TESTONLY packages are installed

INSTALL_TESTONLY_PACKAGES in devstack only impacts .deb/.rpm files
even though most of our test requirements are actually specified
in python. This will do an explicit install of test-requirements.txt
if pip_install is called with args that look like a directory,
and test-requirements.txt exists in that directory.

Change-Id: Id21e282ddc945c819c9c8c4d724658e28bfde785
Merge "install test-reqs when TESTONLY packages are installed"