Allow concurrency to be tweaked for tempest

This adds a knob for tweaking the tempest concurrency, but sets the
default to 4 (what we run at now by default).

This was borrowed from https://review.opensta+ck.org/#/c/58605/3

Change-Id: I90f7da9f6d72858abf58aff35aa2037de4b5e0af
Merge "Allow concurrency to be tweaked for tempest"