"clean_zonefile" option in "targets" section of "pools.yaml" file should be exposed as a charm option

"clean_zonefile" in the "targets" section of the "/etc/designate/pools.yaml" file should be exposed as a configurable option.

At the moment when a zone is deleted from Designate, the designate-bind containers don't delete the zone file in /var/cache/bind for that zone.

Designate has code to get designate-bind to delete files for deleted zones. This was found in /usr/lib/python3/dist-packages/designate/backend/impl_bind9.py on the designate containers:

self._clean_zonefile = strutils.bool_from_string(
                          self.options.get('clean_zonefile', 'false'))

if self._clean_zonefile:
    rndc_op.insert(1, '-clean')

I was able to get Designate to clean the zone files for deleted zones by making the following changes in templates/mitaka/pools.yaml and templates/rocky/pools.yaml:

$ diff designate/templates/mitaka/pools.yaml designate_orig/templates/mitaka/pools.yaml
40d39
< clean_zonefile: true
55d53
< clean_zonefile: true

The updated pools.yaml looks like the following (hostnames and IP addresses have been changed):

ubuntu@juju-a55f84-0-lxd-7:~$ sudo cat /etc/designate/pools.yaml
- id: 794ccc2c-d751-44fe-b57f-8894c9f5c842
  name: default
  description: Pool genergated by Juju

  ns_records:

    - hostname: designate1.example.com.
      priority: 10

    - hostname: designate2.example.com.
      priority: 10

    - hostname: designate3.example.com.
      priority: 10

  nameservers:

    - host: 100.86.0.211
      port: 53

    - host: 100.86.0.212
      port: 53

    - host: 100.86.0.213
      port: 53

  targets:

    - type: bind9
      masters:

        - host: 100.86.0.216
          port: 5354

        - host: 100.86.0.214
          port: 5354

        - host: 100.86.0.215
          port: 5354

      options:
        host: 100.86.0.211
        rndc_host: 100.86.0.211
        rndc_key_file: /etc/designate/rndc.key
        port: 53
        clean_zonefile: true

    - type: bind9
      masters:

        - host: 100.86.0.216
          port: 5354

        - host: 100.86.0.214
          port: 5354

        - host: 100.86.0.215
          port: 5354

      options:
        host: 100.86.0.212
        rndc_host: 100.86.0.212
        rndc_key_file: /etc/designate/rndc.key
        port: 53
        clean_zonefile: true

    - type: bind9
      masters:

        - host: 100.86.0.216
          port: 5354

        - host: 100.86.0.214
          port: 5354

        - host: 100.86.0.215
          port: 5354

      options:
        host: 100.86.0.213
        rndc_host: 100.86.0.213
        rndc_key_file: /etc/designate/rndc.key
        port: 53
        clean_zonefile: true

  also_notifies: []

The goal of this change will be to add this as a configurable option to allow end-users to decide whether or not to delete a zone file when the zone is deleted.