Add nodes text, disk partioning instructions

user-guide/4000-configure-environment.rst -- added "Add nodes" stage to
summary table

user-guide/create-environment/0900-add-nodes.rst -- new file to include the
"Add nodes" stage
user-guide/create-environment/1300-deploy-changes -- broken out into
separate file and moved after disk partitioning

user-guide/create-environment/1000-assign-roles.rst -- added instructions

New image for add-nodes screen
Replaced image for assign-roles screen, disk-partitioning

The new screen shot for assign-roles includes Zabbix; discussion will be
added in a separate Zabbix commit

Closes-Bug: 1352499

Change-Id: I15645ceb18761f7f28950348cb38990bd6f77360