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
provider/manual: look for "no-agent-dir" in StateServerInstances

Look for a more meaningful string, and reject all
other output as an error.
Merge pull request #431 from axw/lp1347715-provider-manual-stateserverinstances-ignore-stderr

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