Add waiting period to Storage Class creation

This commit adds a waiting period until the
storage class is available.

It also fixes a minor bug in the juju add-k8s
command.

This fixes Bug 1155

Change-Id: Ie4d35847bd053c9e4e61f89a481faef5e348f5e2
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>