mgr/cephadm: allow setting prometheus retention time

mgr/cephadm: allow setting prometheus retention time

When we deploy Prometheus server, we don't provide any
ability to define the tsdb retention time - so it defaults to 15d.

This change adds a field that can be passed in a prometheus service
spec that will be passed as an arg to the --storage.tsdb.retention.time
parameter for the prometheus daemon.

Fixes: https://tracker.ceph.com/issues/54308
Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit 91dd03fd648d25773a83fdad311b62b781619fc4)

Conflicts:
	src/pybind/mgr/cephadm/services/monitoring.py
	src/pybind/mgr/cephadm/tests/test_services.py