documentation: warn re: usage of '.' in JUJU_ENV as results in units with same hostname

[JuanJo Ciarlante]

Because of the way juju + cloud-init construct unit's hostname: $JUJU_ENV-machine-$NUM,
if JUJU_ENV has a dot in it, final non-FQDN hostname will be the same for all units -
e.g. with JUJU_ENV=my-service.test1, all units will end named "my-service".

This is obvious once you realize it (my-service.test1-machine-2 doesn't look good as hostname),
but I think it's worth warning/documenting it, as having units with same hostname can
impact charms running services that use it as a unique identifier (e.g. for clustering/peer
relations, unique keys while building conf files after relation hooks, etc).