Introduce configurable defaultParallelism

[lukasherman]

Hello Jet Team, our moderately complex workflow almost exclusively uses localParallelism(1) set on Vertex level, due to inability to set a general default for the local parallelism level, so we only get the hard-coded getCooperativeThreadCount(). Please introduce the defaultLocalParallelism config option on either instance or job level.

With regards Lukas Herman