ci: zabbix: Separate server, agent stages

Separating the Zabbix server and agent playbooks into separate stages
allows better visibility into the time taken for each.

ci/zabbix.jenkinsfile

@@ -45,10 +45,21 @@ pipeline {
             }
         }
 
-        stage('Zabbix') {
+        stage('Zabbix Server') {
             steps {
                 ansiblePlaybook \
-                    playbook: 'zabbix.yml',
+                    playbook: 'zabbix-server.yml',
+                    become: true,
+                    vaultCredentialsId: 'ansible-vault',
+                    extras: '--diff'
+            }
+        }
+
+        stage('Zabbix Agent') {
+            steps {
+                ansiblePlaybook \
+                    playbook: 'zabbix-agent.yml',
+                    limit: '!gw0',
                     become: true,
                     vaultCredentialsId: 'ansible-vault',
                     extras: '--diff'