Make Graylog frontend components optional

This commit makes it optional to get the frontend stuff deployed, by
using two specific variables named graylog_frontend_nginx_enable and
graylog_frontend_keepalived_enable.

Author: rascasoft

graylog.yml

@@ -39,6 +39,10 @@
   tasks:
     - import_role:
         name: 'nginx'
+      when:
+        - graylog_frontend_nginx_enable|bool
 
     - import_role:
         name: 'keepalived'
+      when:
+        - graylog_frontend_keepalived_enable|bool

inventory/lab/group_vars/graylog_frontend.yml

@@ -1,5 +1,9 @@
 ---
 
+# Chose whether or not to enable frontend components
+graylog_frontend_keepalived_enable: true
+graylog_frontend_nginx_enable: true
+
 # Keepalived configuration
 keepalived_vrrp_password: '91514409'
 keepalived_vrrp_vips: