Merge branch 'feature_10922_Stop_start_rebuild' into 'master'

- Support for subscription and notification for VNFD.

- Cache to store subscribers.

## Start, Stop and Rebuild operations over a VDU of a running VNF instance

These three operations involves starting , stopping, and rebuilding of a VDU on a running VNF instance by using the NGUI. 

OSM allows these three operations over VDUs for all supported VIMs.

### Start Operation:

Start operation lets you start one VNF instance at a time and that instance should be in `shutoff` state.

![start operation](assets/500px_start.png)

### Stop Operation:

Stop operation lets you stop one VNF instance at a time and that instance should be in `running` state.

![stop operation](assets/500px_stop.png)

### Rebuild Operation:

Rebuild operation lets you rebuild one VNF instance at a time and in this operation, the instance can be in either `running` or `shutoff` state.

![rebuild operation](assets/500px_rebuild.png)

### Additional Notes

- Each operation is executed independently and only one at a time.

- In the rebuild operation, we are not deleting the target VDU nor we are recreating it. The VDU will be rebuilt by using the existing image, so actual properties of the VDU are preserved like name and IP addresses.

### How to perform operation from UI

From OSM's user action menu select the  operation (start, stop, rebuild),  select the Member VNF index, select the VDU id and Count Index for the target VDU, then select apply. This will trigger the operation.

#### Future roadmap

Implementation of day-1 operation for rebuild operation is in roadmap.