Cinder text changed for Assign roles to nodes

Also added pointer to Zabbix planning doc
and explained the MAC address component of the default node name

Co-Authored-By: Dmitry Borodaenko <dborodaenko@mirantis.com>
Change-Id: I8867755a3e385c0dd6d55f2a8ae3f8be5b4e4251
Closes-Bug: #1398780
Merge "Cinder text changed for Assign roles to nodes"