Need to add ability to clean up juju storage when destroying model

[Frode Nordahl]

In conjunction with work done on bug 1698154 (https://review.openstack.org/#/q/status:open+topic:bug/1698154) we have changed the functional tests to use juju storage.

When destroying a model juju has safeguard against accidental destruction of storage/data, and we need to add the flag `--destroy-storage` in addition to the existing `-y` to the calls to `juju destroy-model` to be able to do pre- and post-test cleanup.

Example of failed test due to inability to clean up:
https://openstack-ci-reports.ubuntu.com/artifacts/test_charm_pipeline_amulet_full/openstack/charm-ceph-osd/571690/6/1605/consoleText.test_charm_single_28679.txt

[Chris MacNaughton (chris.macnaughton)]

https://github.com/openstack-charmers/charm-test-infra/pull/17 is a fix proposed to resolve this