Fix uninstall bug that leaves Juju controller running

During uninstall, the Juju controller was not being destroyed when there
are active models. The `juju destroy-controller` command requires the
additional flag `--destroy-all-models`.

Change-Id: Ie5e3590027907ada08c6e6b2951a6f1ef64d41bf
Signed-off-by: Adam Israel <adam.israel@canonical.com>