mgr/cephadm: resolve IP at 'orch host add' time

mgr/cephadm: resolve IP at 'orch host add' time

We prefer to always have a real IP for hosts in the cluster.  This avoids
a reliance on DNS for most operations.

Perhaps more importantly, it means we are less sensitive to inconsistent
host lookup results, for example due to (1) mismatched /etc/hosts files
between machines, or (2) a lookup of the local hostname that returns
127.0.1.1.

Adjust with_hosts() fixture to take an addr, and adjust tests accordingly.

Signed-off-by: Sage Weil <sage@newdream.net>