Add experimental --parallel functionality

[decentral1se]

Supercedes the spike in #2135. Working towards closing #1702.

I've tried to incorporate comments from last review but as it's all a bit experimental, it was not possible to work them all in. However, I think this is a more elegant solution here. Ready for a review pass.

Q/A Testing much welcome (see "how to test" in #2135 (comment)).

/cc @ssbarnea @seandst @themr0c

[ssbarnea]

a) Make sure user can define environment variable to enable parallel mode.
b) I do not fully understand why the new filter needs any parameters? cannot let the filter implementation take care of adding the uuid when needed?
c) I do have some concerns regarding effects on existing playbooks as people may use ansible fitlering based on hostname, I need to test it to see exactly what happens...

[decentral1se]

Following from #2137 (comment), I've managed to internalise the entire workings of this now with e5ac12c. I use the new MOLECULE_STATE_FILE lookup to store the configuration (instead of passing it in manually into the playbook context) and hijack the from_yaml loading to re-apply the parallelization changes to the molecule.yml at run-time.

(I'll rebase all these fixups into a single commit once the review phase is over ...)