Use pyinvoke and Fabric

[jmfernandez]

Next example shows how to issue commands which can be either remotely or locally run https://stackoverflow.com/a/55704170 based on both pyinvoke and Fabric libraries.

Past the 1.0 milestone, WfExS-backend is going to gain different non-raw execution scenarios, like in-container runs, runs with different users, remote runs through ssh, remote runs through a queue system (first, monolithic, later, spread) and remote runs through GA4GH TES and WES.

A way to seamless integrate this is first transitioning to use both pyinvoke and Fabric, so local and ssh executions are seamless integrated, and then trying to extend it to the other execution environments.