cephadm: change ceph & exporter to customize_container_mounts method
Unlike the other types Ceph and CephExporter share the underlying
method. There was no other use of get_container_mounts on the class
so it could be converted to be customize_container_mounts.
Because there's an extra arg that passes from get_container_mounts
top-level function to Ceph.get_ceph_mounts, that function was not
changed.
Signed-off-by: John Mulligan <jmulligan@redhat.com>