provider/manual: don't verify bootstrap-host from inside env

StateServerInstances was attempting to verify the bootstrap
host by SSHing to itself; this fails if it does not have
authorized_keys set up appropriately. We use use-sshstorage
to detect whether we're inside the bootstrap host, and then
elide the verification if so.

Fixes https://bugs.launchpad.net/juju-core/+bug/1347715
Using the existence of SSH storage to determine if you're on the bootstrap host is a little hacky. At least you've got a good comment there.