xci: Fix checkout location for OpenStack-Ansible

xci: Fix checkout location for OpenStack-Ansible

The OPENSTACK_OSA_PATH only makes sense on localhost. As such, when we
use it on playbooks that operate on remote hosts, the result is not
predictable. However, we rsync the entire releng-xci repository to the
opfnv host so we can make everything predictable by simply clone
everything in advance in the .cache directory. That directory is then
rsync'd to the opnfv host. As such, we can repurpose the
OPENSTACK_OSA_PATH to point to the path into the OPNFV host. Moreover,
all external repositories are being cloned to .cache/repos so we can
eliminate some variables in order to simplify the code. Finally, we
bring back the ability to use an external OSA repository for
development purposes.

Change-Id: Ieef3e22ae2085f6735185634d555cfc0d4b69b39
Signed-off-by: Markos Chandras <mchandras@suse.de>