openstack: get user-data path relative to the sources

openstack: get user-data path relative to the sources

The user-data files are in the sources, in the teuthology/openstack
directory. By default the template path being used is
teuthology/openstack/openstack-{os_type}-{os_version}-user-data.txt and
assumes teuthology-suite / teuthology-openstack is run from the root of
the source directory.

If the user-data path is relative, make it absolute based on the
directory in which the openstack module is found.

http://tracker.ceph.com/issues/13416 Fixes: #13416

Signed-off-by: Loic Dachary <ldachary@redhat.com>