storageTests: Configure via environment variables

These are used as permutations for the test. We want to automate the
test and call it with whatever storage type(s) and metadata
version(s).

Change-Id: I84f1ee57e43d921ae38afaabddecca9f38dd2687
Signed-off-by: Vered Volansky <vvolansk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23259
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>