Several fixes to the charmed installer

- fix bug 1394: update images defining a default TAG (=9)
- fix bug 1401: add --advertise-address option to kube-apiserver in
microk8s
- fix bug 1405
- fix bug 1406: pin the version of the osmclient snap. Add the logic to
install/refresh the snap to the proper version. Add channel preference
(stable -> candidate -> beta -> edge), so the installers tries first
with stable, but if it does not exist, then tries with candidate, etc.

Change-Id: I02edaaeed81e3b06ed328e81ecab2bf71d42c628
Signed-off-by: David Garcia <david.garcia@canonical.com>