Add parallel local image building.

[tmc]

#248 added parallel building to gcb+kaniko, but not local builds.

Expected behavior

That local builds use InParallel

Actual behavior

Local builds use InSequence

[balopat]

I would love to have this somehow.

Thoughts:

This is only for local builds.
On cluster and gcb builds we have parallel builds.

1. We experimented with this before for local builds but things were breaking back then . @dgageot maybe it's worth having another look now
2. There are people who depend on sequenced builds - they use it as a poor man's dependency management solution.