dashboard: add location field to NVMeoF namespace and gateway group APIs

dashboard: add location field to NVMeoF namespace and gateway group APIs

Namespace location:
- Add location field to Namespace model in nvmeof.py
- Add location parameter to PATCH /api/nvmeof/subsystem/{nqn}/namespace/{nsid}
- Location can now be retrieved via GET and set via PATCH

Gateway group locations:
- Add locations array to gateway group endpoint response
- Extract locations from all gateways in a service group
- Add _get_gateway_locations() helper method using nvme-gw show command
- Locations appear in placement.locations for each service

Signed-off-by: Victoria Mackie <victoriam@uk.ibm.com>