1000: Fix authentication when deleting service

This fixes 1000 where the creation of k8sclusters
was failing once a k8scluster had been deleted. This
was due to N2VC not logging out correctly from the
juju controller. Error handling was added to the
add_model funciton.

https://osm.etsi.org/bugzilla/show_bug.cgi?id=1000

Change-Id: I144ddf7ed16a3aa65e145459e49f41b9fd936353
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>