Update k8s tests to check cluster creation inside testsuite

This change moves the check of the cluster creation to the testsuite.
In that way, the test can change depending on whether the juju controller
exists or doesn't exist.

Change-Id: I072c27061745ca8d8766d0dc1ded71a252cceef2
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>