manual provisioning should ignore ssh warnings when detecting presence of /etc/init/juju*

[Andrew Wilkins]

ahasenack reported an issue with manual provisioning, where juju was incorrectly detecting the presence of a machine agent on the target host.

juju executes 'ssh <host> "ls /etc/init/ | grep juju.*\\.conf || exit 0"'; we are currently checking that both stdout&stderr are non-empty. ssh may log a warning to stderr if it automatically adds a known host. We should ignore stderr.