Avoid execution of tests in run_test.sh if no tag is provided

The idea of this commit is to enable the use of run_test.sh for
other purposes like running TST010 conformance tests or register VIM
and K8s clusters, so that the VIM and cluster registration can be
decoupled. In that way, daily jobs could run several tests, one after
the other, with different VIMs.

Change-Id: If0791f9562d23a861f8491c144cdc7f539efec46
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>