remove-machine: tell user of unit/storage changes

[axw]

Description of change

Introduce the MachineManager.(Force)DestroyMachines
methods, and use them in "juju remove-machine". We
will continue to use the existing Client facade
methods when the MachineManager facade is not new
enough.

Using the new methods, we print out the names of
units and storage that will be destroyed or detached
when removing the machine.

QA steps

(with juju 2.0)

1. juju bootstrap localhost
   (with juju from this branch)
2. juju deploy ubuntu
3. juju remove-machine 0 --force
4. juju upgrade-juju -m controller --build-agent
5. juju add-unit ubuntu
6. juju remove-machine 1
   (fails, because there's a unit assigned)
7. juju remove-machine --force 1
   prints out:

removing machine 1
- will remove unit ubuntu/1

8. juju deploy postgresql --storage pgdata=rootfs
9. juju remove-machine --force 2
   prints out:

removing machine 2
- will remove unit postgresql/0
- will remove storage pgdata/0

Documentation changes

Affects output of remove-machine command, so examples may need updating.

Bug reference

None.

[wallyworld]

We'll need to get CI tests updated. Can you add a card to the board and we'll talk to Burton.

[axw]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/10475

[axw]

$$merge$$

[jujubot]

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