mgr/cephadm: add custom choose_next_action to ceph exporter service

mgr/cephadm: add custom choose_next_action to ceph exporter service

The ceph exporter service (and similar monitoring stack services)
need to detect if andy dependencies in the mgmt stack support services have
changed and be redeployed if so.

Update the ceph exporter service that makes use of a common function
for checking for this need. A common function will be used instead of
messing around with inheritance because I'm simply not brave enough
to look at doing that and I know a function provides common
implementation without side-effecting the class hierarchy.

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