Merge branch 'Autohealing' into 'master'

Any of the VMs can be accessed through SSH (credential: `ubuntu`/`osm2021`) to further monitor (with `htop`, for example), and there is an HAProxy UI at port `http://[HAProxy_IP]:32700` (credential: `osm`/`osm2018`)

### Autohealing

#### Reference diagram

The following diagram summarizes the feature:

![Diagram explaining auto-healing support](assets/800px-Osm_healing.png)

- Healing descriptors can be included and be tied to automatic reaction to VM metric thresholds.

- An internal alarm manager has been added to MON through the 'mon-evaluator' module, so VM metrics can trigger threshold-violation alarms when VM is in `ERROR/DELETE` state and perform healing actions.

#### Healing Descriptor

The healing descriptor is part of a VNFD. Like the example below shows, it mainly specifies:

- The VDU to be healed (`healing-policy:vdu-id`)

- The healing recovery option (`action-on-recovery`)

- The minimum time it should pass between healing operations (`cooldown-time`)

- To run day1 primitives for VDU (`day1`)

```yaml

healing-aspect:

-   id: autoheal_vnfd-VM_autoheal

    healing-policy:

    - vdu-id: autoheal_vnfd-VM

      event-name: heal-alarm

      recovery-type: automatic

      action-on-recovery: REDEPLOY_ONLY

      cooldown-time: 180

      day1: false

```

#### Example

Get the descriptors:

```bash

git clone https://osm.etsi.org/gitlab/vnf-onboarding/osm-packages.git

```

Onboard them:

```bash

cd osm-packages

osm vnfpkg-create autoheal_vnf

osm nspkg-create autoheal_ns

```

Launch the NS:

```bash

osm ns-create --ns_name heal --nsd_name autoheal_nsd --vim_account <VIM_ACCOUNT_NAME>|<VIM_ACCOUNT_ID>

osm ns-list

osm ns-show heal

```

#### Testing:

1. To ensure NS is instantiated successfully, check metrics at Prometheus, visit `http://[OSM_IP]:9091` and look for `osm_vm_status`. Metric value should be '1'.

2. Run the following openstack commands to induce and error or delete a VM:

```bash

# To test healing in error state, induce error state in vm

openstack server set --state error <server-id>

# To test healing in deleted state, delete the vm

openstack server delete <server-id>

```

3. Check metrics at Prometheus, visit `http://[OSM_IP]:9091` and look for `osm_vm_status`. Metric value should be '0'.

4. Heal operation will be triggered at POL and VM respawn will happen.

## Using Network Slices

In order to illustrate better how network slicing works in OSM, it will be discussed in the context of a running example.