Support parallel builds with GC3Pie (#1006, WIP)

This is meant to be the implementation of #1006.

It's currently work-in-progress and in very early stages too.
for an example, look at the module naming scheme code:
https://github.com/hpcugent/easybuild-framework/blob/master/easybuild/tools/module_naming_scheme/mns.py for the API
https://github.com/hpcugent/easybuild-framework/blob/master/easybuild/tools/module_naming_scheme/easybuild_mns.py one valid MNS
https://github.com/hpcugent/easybuild-framework/blob/master/easybuild/tools/module_naming_scheme/utilities.py#L68 : a function to gather all valid ones