mgr/cephadm/iscsi: simplify the dashboard check

mgr/cephadm/iscsi: simplify the dashboard check

We don't need to run an extra command (mgr module ls) to obtain the mgr
modules list since we already have this information in the mgr_map.
This workflow is already done for the monitoring stack or for configuring
the iscsi integration within the dashboard (during creation) via the
config_dashboard method.

The mgr_map is mocked in the tests with the dashboard module enabled so we
don't need _mon_command_mock_mgr_module_ls anymore.

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>