pybind/mgr/mgr_module: make get_mgr_ip() return mgr's IP from mgrmap

pybind/mgr/mgr_module: make get_mgr_ip() return mgr's IP from mgrmap

The previous approach was convoluted: we tried to do a DNS lookup on the
hostname, which would fail if /etc/hosts had an entry.  Which, with podman,
it does.  And the IP it has will vary in all sorts of weird ways.  For
example, CNI on my host means that I get a dynamic address in 10.88.0.0/24.

Avoid all of that nonsense and use the IP that is in the mgrmap.  There
may be multiple IPs (v2 + v1, or maybe even IPv4 + v6 in the future); in
that case, use the first one.

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