cephadm: stop directly using Ceph.daemons property

cephadm: stop directly using Ceph.daemons property

The Ceph.daemons property has two unfortunate behaviors: most important,
it includes ceph-exporter which causes the other CephExporter class to
be over-shadowed the DaemonForms mechanism. Second, it couples all
functions that want to know the names of ceph daemon types to the Ceph
class preventing future refactoring of that class.

Break the existing coupling by adding a new `ceph_daemons` function
similar to `get_supported_daemons` but returning the same value that
Ceph.daemons used to provide. This will permit future fixes and
improvements.

Signed-off-by: John Mulligan <jmulligan@redhat.com>