mgr/cephadm: allow nvmeof group assignment for NVMe-oF services

NVMe-oF services created in older releases may have a service_id that does
not include the gateway group name, because those services could be created
without a group.
The current validation requires an NVMe-oF service_id to end with
the configured group name when spec.group is set. This is correct for new
services, but it blocks the documented upgrade flow for existing legacy
services where the user exports the existing spec, adds spec.group, and
applies it back.

Allow this narrow update path when the service already exists, the stored
spec has no group, and the incoming spec keeps the same service_id while
adding a non-empty group. Keep the existing validation for new services and
for services that already have a group, so duplicate or misleading group
configurations are still rejected.

Signed-off-by: Kushal Deb <Kushal.Deb@ibm.com>