Put back the old non-parallelized nova boot path; option for future.

I won't get the Neutron lock bug solved in time for this eval churn, so
for now, undo the path where we launch all the slaves via a single 'nova
boot' call.  This caused lockups in Neutron after N slaves had been
created; cause unknown.

Doing a call per VM probably isn't much slower in the long run, anyway.
1 job for master in 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #745
testing

00:00:02