Currently when a string is passed to the parallel engine
it will only know how to create a process or a native thread
based executor. The futurist library also supports making
a green thread based executor, so support creating it.
This will save glance some code that they have to create
a executor based on different options (one of those is a
green option).
Change-Id: I15c164a38b4445d28eb6062aed6c56cce0e0364b