Add HA option and minor fixes

This commit adds the --ha option which will
install the osm-ha bundle instead of the standard
osm bundle.

Additionally this commit includes the following
fixes:
- change the use of route to ip route which is
preinstalled in Ubuntu Desktop also.
- Change final command of post install osmclient
message
- Change post install osmclient message to be
only shown during install and not uninstall.
- Fix the deletion of the wrong model in uninstall
- Pining Juju snap to 2.7/stable

Change-Id: Iccf3bda7ec2f9a2279e5535d819e2a1968a59687
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>