mgr/cephadm: make HostAssignment make sense

mgr/cephadm: make HostAssignment make sense

- if hosts are passed, use those.
- if all_hosts=true, use all hosts.

Otherwise, build a set of hosts based on the labels--either explicit or
implied.  If there's no label, use all hosts.

If there is a count, use a subset of candidate hosts.  If there was no
label and there is no count, fail.

If count and hosts are both provided, then we either (1) use the hosts
as the candidate list and place among them (if len(hosts) >= count), or
(2) ensure that any result includes *at least* the provided hosts.

Add tests that (I'm pretty sure) cover cases!

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