install-autotest-server.sh: Use same branch for tests as autotest.

Signed-off-by: Vinson Lee <vlee@twitter.com>
contrib/install-autotest-server.sh: Make test branches configurable

If we are testing autotest from our own git repo and branch,
we want to make sure an appropriate upstream branch for the
tests submodules will be pulled. So, give that option to the
user.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>