mgr/dashboard: connect-rgw: rename to set-rgw-credentials; refactoring

mgr/dashboard: connect-rgw: rename to set-rgw-credentials; refactoring

- Rename the dashboard command to better reflect its behavior.
- Rename '_radosgw_admin' method to 'send_rgwadmin_command' for consistency with
  'send_mon_command' and move it to the mgr_module.py .
- Cleanup: remove unneeded rgw settings.
- Better error handling and test coverage.

Fixes: https://tracker.ceph.com/issues/44605
Signed-off-by: Alfonso Martínez <almartin@redhat.com>
(cherry picked from commit 6e20ef1dd35f3681d14cd4e08ca63eb20edc2c88)