ceph-mon: bind to public_addrv if set (before public_addr or public_network)

ceph-mon: bind to public_addrv if set (before public_addr or public_network)

This lets you specify a precise addrvec for the monitor to bind to,
instead of inferring a v1 or v2 address from the port number, or assuming
both with default ports, or v2 only with an unrecognized port.

I unfortunately don't quite remember why I didn't make public_addr a
ADDRVEC option way back in nautilus--I only remmeber that it was going to
cause problems.  Adding a separate addrv option here is safe.

Signed-off-by: Sage Weil <sage@redhat.com>