Support for zabbix

-added required entries in openstack.yaml
-changed deployment ordering so zabbix will always be deployed first
-added UI check to prevent assigning multiple zabbix servers in an environment
-added a link to zabbix dashboard after a successful deployment w/ zabbix

Change-Id: Ia417f7cc64d0027532d0dadc0553bcaa986abcff
Implements: blueprint monitoring-system