Automatically detect OSM_HOSTNAME

I don't know how complicated is this:

When executing the osmclient.osm command, check if the OSM_HOSTNAME exists. If it exists, then continue. If not, check if the following command gives an IP (means that Charmed OSM is installed). <-- Autoexport the OSM_HOSTNAME to use that IP.

$ juju status nbi-k8s --format json | jq -r '.applications["nbi-k8s"].address'
10.152.183.241

If the command gives an error, then print an error saying that the OSM_HOSTNAME variable is required.