add TEMPEST_LARGE_OPS_NUMBER to option

The tempest scenario.large_ops test tries to catch any performance issues
when running large numbers of operations at once, in this case launching
instances.

Set to 0 by default, to maintain the current default, but add this as an
option so this can be set via devstacks localrc.

Part of regression test for bug 1199433

Change-Id: I459717b849f9b6d180a7956c0ff470cefe7c1ff6
Merge "add TEMPEST_LARGE_OPS_NUMBER to option"