Parallelize (only) build stage when remote build is used

[ykis-0-0]

Since we now have the option remoteBuild, I think it would be rather feasible to parallelize the build of the profiles on hosts.

It may be better to keep activation serial, however for building the profiles, since doesn't cause any dependency issues, I suppose it'll beneficial to parallelize the builds when built on remote hosts.

[rvem]

This idea was expressed already in #46 (comment) :)
I agree that it'd be useful to have. Perhaps, we'll get to it eventually, but sadly I can't give any estimates at the moment

[ykis-0-0]

Oh, sorry for raising the redundant issue, I shall close this in favor for the more general discussion