Force-destroy machine if manual provisioning fails

If manual provisioning fails, we leave a machine
record in state that can only be removed with
"juju destroy-machine --force". We change the
manual provisioning code to call destroy the machine
with force if it fails.

(Partially) fixes https://bugs.launchpad.net/juju-core/+bug/1356886
Merge pull request #863 from axw/lp1356886-manual-fail-forcedestroy

Force-destroy machine if manual provisioning fails

If manual provisioning fails, we leave a machine
record in state that can only be removed with
"juju destroy-machine --force". We change the
manual provisioning code to call destroy the machine
with force if it fails.

(Partially) fixes https://bugs.launchpad.net/juju-core/+bug/1356886