Increase reserved memory for computes when enabling DVR
Increase reserved memory for computes when enabling DVR

This patch adds an example increased value for NovaReservedHostMemory
and some documentation around tuning this value when DVR is enabled.

Closes-Bug: #1630583

Change-Id: I2718d72d307a1c90061606e5f36c96f964cd2fb5

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates