Adding documentation for feature 10936 keeping persistent volume of VNF

Where the `id` of the `affinity-or-anti-affinity-group` is the one in the descriptor, and the `vim-affinity-group-id` is the guid of the existing server-group in Openstack to be used (instead of being created).

### Keeping Persistent Volumes

OSM supports three types of volumes: persistent, swap and ephemeral. Swap and ephemeral volumes are deleted together with the virtual machine. 

Persistent volumes are used as an root disk or ordinary disk and could be kept in the Openstack Cloud environment upon virtual machine deletion by setting `keep-volume` flag `true` under `vdu-storage-requirements` in the VNFD. If the `keep-volume` is set to `false` or is not included in the descriptor, persistent volume is deleted together with virtual machine. A sample descriptor which keeps persistent volumes is given as follows:

```yaml

vnfd:

  description: A basic VNF descriptor w/ one VDU and several volumes, keeping persistent volume

  df:

  - id: default-df

    instantiation-level:

    - id: default-instantiation-level

      vdu-level:

      - number-of-instances: 1

        vdu-id: keep-persistent-vol-VM

    vdu-profile:

    - id: keep-persistent-vol-VM

      min-number-of-instances: 1

  id: keep_persistent-volumes-vnf

  mgmt-cp: vnf-mgmt-ext

  product-name: keep_persistent-volumes-vnf

  vdu:

  - id: keep-persistent-vol-VM

    name: keep-persistent-vol-VM

    sw-image-desc: ubuntu20.04

    alternative-sw-image-desc:

    - ubuntu20.04-aws

    - ubuntu20.04-azure

    virtual-compute-desc: keep-persistent-vol-VM-compute

    virtual-storage-desc:

    - root-volume

    - persistent-volume

    - ephemeral-volume

  version: 1.0

  virtual-storage-desc:

  - id: root-volume

    type-of-storage: persistent-storage

    size-of-storage: 10

    vdu-storage-requirements:

      - key: keep-volume

        value: 'true'

  - id: persistent-volume

    type-of-storage: persistent-storage

    size-of-storage: 1

    vdu-storage-requirements:

      - key: keep-volume

        value: 'true'

  - id: ephemeral-volume

    type-of-storage: ephemeral-storage

    size-of-storage: 2

```

An existing persistent volume could be passed as an instantiation parameter by identifing the name of `volume` and `vim-volume-id` which is exact volume ID in the Openstack Cloud. `vim-volume-id` is only accepted as an instantiation parameter, it could not be provided in the descriptor. If the `vim-volume-id` is provided as a persistent volume, new persistent volume is not created, but reused. Existing volumes which are provided with `vim-volume-id` parameter are always kept without checking `keep-volume` flag, when the Network Service instance is deleted. The following example shows the syntax:

```yaml

--config '{ vnf: [ {member-vnf-index: vnf-persistent-volumes, vdu: [ {id: keep-persistent-vol-VM, volume: [{"name": root-volume, vim-volume-id: 53c485d0-7f32-4675-919d-a3ccaf655629}, {"name": persistent-volume, vim-volume-id: 4391a6af-6e00-470c-960f-73213840431e}] } ] } ] }'

```

Where the `name` of the `persistent-storage` is the one in the descriptor, and the `vim-volume-id` is the ID of volume in Openstack to be used (instead of being created).

## Understanding Day-1 and Day-2 Operations

VNF configuration is done in three "days":