state/apiserver/client: don't send containers to StopInstances

The DestroyEnvironment API attempts to destroy all non-manager,
non-manual instances. We should also ignore containers here.

Fixes https://bugs.launchpad.net/juju-core/+bug/1325830
Merge pull request #69 from axw/lp1325830-maas-destroy-containers

state/apiserver/client: don't send containers to StopInstances

The DestroyEnvironment API attempts to destroy all non-manager,
non-manual instances. We should also ignore containers here.

Fixes https://bugs.launchpad.net/juju-core/+bug/1325830