try this

roles/nagios_server/files/nagios/commands/mailman.cfg

@@ -0,0 +1,29 @@
+################################################################################
+# COMMAND DEFINITIONS
+#
+# SYNTAX:
+#
+#	define command{
+#               template      <templatename>
+#		name          <objectname>
+#               command_name  <commandname>
+#               command_line  <commandline>
+#               }
+#
+# WHERE:
+#
+# <templatename> = object name of another command definition that should be
+#                  used as a template for this definition (optional)
+# <objectname>   = object name of command definition, referenced by other
+#                  command definitions that use it as a template (optional)
+# <commandname>  = name of the command, as recognized/used by Nagios
+# <commandline>  = command line
+#
+################################################################################
+
+# 'check_mailman_api'
+define command{
+        command_name    check_mailman_api
+        command_line    $USER1$/check_http -H localhost -p 8001 -u /3.0
+}
+

roles/nagios_server/tasks/main.yml

@@ -137,6 +137,7 @@
     - httpd.cfg
     - koji.cfg
     - local.cfg
+    - mailman.cfg
     - misc.cfg
     - notify.cfg
     - nrpe.cfg