Use NS uuid when creating a juju k8s model

This commit fixes bug 972, previously it was using the namespace
value that was always assigned to kube-system. This caused an error
in Juju due to kube-system already existing.
https://osm.etsi.org/bugzilla/show_bug.cgi?id=972



Change-Id: I840d65ef9991f64eba42a205900d42cb0a9bbeef
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>