mgr/cephadm/schedule: choose an IP from a subnet list

mgr/cephadm/schedule: choose an IP from a subnet list

Choose an IP from the subnet list provided by the ServiceSpec.

A few caveats:
- we ignore hosts that don't have IPs in the given subnet
- the subnet matching is STRICT.  That is, the CIDR name has to exactly
match what is configured on the host.  That means you can't just say 10/8
to match any 10.whatever addres--you need the exactly network on the host
(e.g, 10.1.2.0/24).
- If you modify a servicespec and change the networks when there are
already deployed daemons, we will try to deploy the new instances on
the same ports but bound to a specific IP instead of *.  Which will fail.
You need to remove the service first, or remove the old daemons manually
so that creating new ones will succeed.

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