Autodetect if it is possible to ssh to the openstack vm skipping proxy

[sergiocazzolato]

The idea is to bypass the proxy configuratio when it is not needed, so we don't need to duplicate the backends configuration

[maykathm]

Looks good for our context. My only hesitation is that if I set a proxy in spread, I assume that proxy will be used. This is changing it to a "only use if you really need to" approach. I think the approach change is fine, but perhaps we should declare this behavior in the readme or somewhere