mgr/dashboard: fix group name bugs in the nvmeof API

mgr/dashboard: fix group name bugs in the nvmeof API

there are 2 issues

1. in cephadm, i was always using the first daemon to populate the group
   in all the services for the dashboard config.

2. in the API, if there are more than 1 gateways listed in the config,
   rather than chosing a random gateway from the group, raise an
   exception and warn user to specify the gw_group parameter in the api
   request

Fixes: https://tracker.ceph.com/issues/68463
Signed-off-by: Nizamudeen A <nia@redhat.com>