Zabbix documentation

planning/4450-monitoring -- main section for the monitoring tools.
Will eventurally introduce Ceilometer/MongoDB and Zabbix then source in
the individual files with details.

planning/monitoring/zabbix.rst -- information about planning for zabbix

Deleted operations/8550-zabbix -- how to use the dashboard and set templates

ref-arch/8100-zabbix.rst -- internals of Zabbix implementation

user-guide/config-environment/8000-zabbix-access.rst presents the
Settings page used to reset the Zabbix password.

terminology/z/zabbix -- terminology article

Note:  The screen where one assigns Zabbix to a role is in a different
commit so does not show in this commit at this time.

Change-Id: I6bac22838ccee596327508cb69df59b0aa49e7ee