Allow worker count to be specified when no executor provided

When a multi-threaded engine is used it is nice to be able
to inform the engine how many workers should be created when
it creates its own executor. To allow this to be possible accept
a new keywork argument that can be used to set this value.

Change-Id: I0095d548249372440abbcf9b5c3b8fa841ca0ea9
Merge "Allow worker count to be specified when no executor provided"