provider/manual: ignore stderr when checking for existence of agents dir

We run an ssh command in StateServerInstances to verify that the bootstrap
machine is provisioned. We were accidentally considering that non-empty
stderr constituted a lack of environment, while we should only have been
considering stdout.

Fixes https://bugs.launchpad.net/juju-core/+bug/1347715
Done. Also changed the code to reject other output, as discussed live.