Run pipeline stage blueprints in parallel with vcli

[VJftw]

A nice small enhancement to the vcli would be to run pipeline stage blueprints in parallel instead of sequentially. i.e. given a plan of

~~~~~~~~~~~~~~~~~~~~~
~ Pipeline: default ~
~~~~~~~~~~~~~~~~~~~~~
 Stage 1
   -> backend/test: Tests the Backend source
 Stage 2
   -> backend/builder/publish: Builds and publishes Builder           
   -> backend/cli/publish: Builds and publishes CLI

Both backend/builder/publish and backend/cli/publish should be ran in parallel instead of sequentially. We should add a flag to limit the number of parallel blueprints (default: number of CPU cores)