roles/graylog: Add Graylog server deployment

The *graylog* role installs Graylog from the *graylog2.org* Yum
repository and manages basic server configuration.  It augments the
default systemd unit to provide the `CAP_NET_BIND_SERVICE` capability to
the Graylog server process via ambient capabilities, thereby allowing
the server to bind to the privileged Syslog UDP port.

roles/graylog/handlers/main.yml

@@ -0,0 +1,12 @@
+- name: save firewalld configuration
+  command: firewall-cmd --runtime-to-permanent
+- name: reload systemd
+  command: systemctl daemon-reload
+- name: restart graylog
+  service:
+    name=graylog-server
+    state=restarted
+- name: reload httpd
+  service:
+    name=httpd
+    state=reloaded