Cut tempest concurrency in half

While a group of us were investigating the top gate bugs today, at least
half of them are due to performance issues where nova isn't able to keep
up.  This change is to test the theory that dialing back the test load
will increase reliability of the gate.

We also have a lot of good data and ideas on how we can improve nova
performance (in particular, nova-network).  Hopefully we could turn this
back up in the future.

Change-Id: I2338ebf5df8bced935e9ed9b0ebd2d4e859b5dbe
Merge "Cut tempest concurrency in half"