mgr/telemetry: use cluster-provided timestamp unmolested

mgr/telemetry: use cluster-provided timestamp unmolested

The cluster stamp is now ISO 8601; just use that.

(The isoformat() puts a : in +hh:mm the timezone offset, which is slightly
different than what Ceph does; just pass Ceph's value through for
consistency.)

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 383006a5cc2da3e0b643a9dc600e75e1ce088bd6)

Conflicts:
	src/pybind/mgr/telemetry/module.py
          Due to missing scaffolding that exists in master but was not
          backported to luminous.