disconnect novnc from gitbase

It turns out that grenade sets gitbase, however some of our
repositories don't have git.openstack.org repos. This causes a
really interesting failure case with grenade because novnc checkout
fails, then it does a checkout to master of nova.

Change-Id: I820b661dacee019928200935ae10d1e36d7b821b
don't leave ourselves inside git directories

to ensure that we don't accidentally do the wrong thing to the
wrong git directory, make sure we return to the original directory
after doing a checkout.

Change-Id: I15d97bee86388f12ee1768123cd8fece78157ba6