Make rmmod ipv6 handler more modular.

Although it seems like EL7 builds ipv6 into the kernel, let's check
that it is indeed the case.  If so we'll use rmmod, if not we'll
just continue.

install/roles/logstash/tasks/main.yml

@@ -115,11 +115,16 @@
     - net.ipv6.conf.default.disable_ipv6
     - net.ipv6.conf.lo.disable_ipv6
 
-- name: Disable IPv6 via modprobe
+- name: Determine if ipv6 is a loadable module
+  command: /sbin/lsmod | grep ipv6 | grep -v nf
+  register: ipv6loadable
+  ignore_errors: true
+
+- name: Disable IPv6 via rmmod
   lineinfile: "dest=/etc/modprobe.conf line='install ipv6 /bin/true' create=yes"
   notify:
     - rmmodipv6
-  when: ansible_os_family == 'RedHat'
+  when: ansible_os_family == 'RedHat' and ipv6loadable.rc != 1
 
 - include: ipv6-grub-disable.yml