Remove openssh communicator modifications

[antonio]

The op_ssh communicator should be integrate as a plugin inside the DRM4G.

All the ad-hoc modifications made into the code, are quite controversial for development.

If the DRM4G plugin model doesn't fit into this, then a change should be made to the model.

Also the openssh communicator needs to be back to the design table.

[minondoa]

I managed to restore the code to how it was while maintaining the op_ssh communicator, with one exception. The entry in the dictionary "COMMUNICATORS" in the drm4g's module "__init__.py" is still needed:

COMMUNICATORS = {
                 "ssh"    : "drm4g.communicators.ssh",
                 "pk_ssh" : "drm4g.communicators.ssh",
                 "op_ssh" : "drm4g.communicators.openssh",
                 "local"  : "drm4g.communicators.local",
                 }