Issue https://github.com/JuliaLang/julia/issues/6264 related fixes

I have implemented feature request of https://github.com/JuliaLang/julia/issues/6264

Now we are able to create heterogeneous clusters where some nodes might not be visible to each other and one should initiate an SSH tunnel between them.

One additional bud is fixed which is related to https://github.com/JuliaLang/julia/issues/6256:
1. Execution of ```addporcs(n)``` fails when some nodes where already added via SSH tunnel.