Fix bug 1267: Destroy machines that are pending

Fix bug 1267: Destroy machines that are pending

When osm destroys a NS, and a native charm hasn't been able
to finish the provisioning of a machine, it will be in a pending state.
As it was seen in the bug 1267, when a model is destroyed and machines
are pending, the deletion of the model will get stuck.

This patch solves that by checking (before deleting the model) if there
are any manually provisioned machines in the juju model (native charms)
in a pending state. If there are, it will destroy them so that the deletion
of the model won't get stuck.

Change-Id: Ia3f6ae7390ccc2bb5d4874d7d08fd446019b0901
Signed-off-by: David Garcia <david.garcia@canonical.com>