Configure large-ops test and add a larger one for testing

This uses the devstack-gate change to allow setting the size of
the large-ops test. It keeps the existing large-ops test at 50,
but also adds a new one at 175 in the experimental queue for us to
do some testing with.

Change-Id: I98baec6e0890f6b10afbd9858ded933f59db9725
Merge "Configure large-ops test and add a larger one for testing"