mgr/telemetry: refactor/enhance CLI commands

mgr/telemetry: refactor/enhance CLI commands

1. Move the implementations of the `on` and `off`
command into dedicated methods so that they can
be accessed by other mgr modules via `mgr.remote`.

2. Add a `show-all` command to return the report
and device report at once. Also add a method
`get_report` to make the reports accessible for
other mgr modules as mentioned above.

3. Add two  minor changes and a note where the
gathered data can be found to the Telemetry docu
file.

Signed-off-by: Tatjana Dehler <tdehler@suse.com>