state: introduce DestroyModelParams

[axw]

Description of change

We consolidate Model.Destroy and Model.DestroyIncludingHosted
into a single method, Model.Destroy, and introduce the
DestoryModelParams struct to control its behaviour. There are
two fields for now:

• DestroyHostedModels. If this is true, then the behaviour
  is the same as the removed Model.DestroyIncludingHosted
  method; otherwise the behaviour is the same as the replaced
  Model.Destroy method.
• DestroyStorage. If this is nil, and there is any persistent
  storage in the model, then destroying the model will fail
  with an error that satisfies state.IsHasPersistentStorageError.
  If the field's value is true, then all storage will be
  destroyed. If the field's value is false, then the storage
  will be released/disassociated from the model/controller.

To implement these changes, we have to extend the cleanup doc
with cleanup-specific arguments.

To check whether storage is persistent in hosted models, we
have had to change various volume and filesystem query functions
to operate on a database, instead of a state object. This
avoids an expensive State.ForModel call.

In a followup we'll extend the API and CLI to force the user to choose
between destroying or releasing storage when using the destroy-model
and destroy-controller commands.

QA steps

1. juju bootstrap localhost
2. juju deploy postgresql --storage pgdata=lxd
3. juju destroy-controller -y localhost --destroy-all-models

Controller should be destroyed, along with storage, as before.

Documentation changes

Not yet.

Bug reference

None.

[wallyworld]

clean this up?

[axw]

oops, thanks

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju