mgr/telemetry: add commands to enable/disable channels

mgr/telemetry: add commands to enable/disable channels

Currently we enable/disable a telemetry channel via CLI with:
  `ceph config set mgr mgr/telemetry/channel_basic true`
  `ceph config set mgr mgr/telemetry/channel_crash false`

We can now do this with:
  `ceph telemetry enable channel basic`
  `ceph telemetry disable channel crash`

We allow enabling / disabling lists of channels:
  `ceph telemetry enable channel basic device crash perf`
  `ceph telemetry disable channel basic device crash perf`

Please note, telemetry should be on for these commands to take effect.

Signed-off-by: Yaarit Hatuka <yaarit@redhat.com>