Set NODE_CNT to 15 and add OVERCLOUD_CONTROLSCALE

Adding the ability to specify multiple control nodes for merge.py. This
requires additional testenv node definitions so setting there to be a
default of 15 nodes defined. This should prevent us from re-deploying CI
testenv's when the number of nodes needed changes in the future.

This change depends on I70ef83f35064ab388bdc7e1a6da62b6585580010

Change-Id: Icff5e728148f2744f3fc7c6e99d744be9fda180b
Merge "Set NODE_CNT to 15 and add OVERCLOUD_CONTROLSCALE"