add missing ClusterApp mixin to SSHClusterLauncher

fixes AttributeError on profile_dir in SSHProxyEngineSetLauncher

also adds SSHProxyEngineSetLauncher to ssh launcher collection,
which adds it to the generated config files, help output, etc.
basic launcher tests

doesn't do much at the moment,
but at least makes basic exercise of constructing objects
Merge pull request #3351 from minrk/sshproxy

start basic tests of ipcluster Launchers