Switch docs.o.o to worker MPM

This is a follow-on from I064b81076bcfcb200e0990627fc1aa0be8bbf058

We see the same issues with "scoreboard is full" and segfaults/dropped
connections on this host.  It's a semi-frequent issue in

logs.o.o seems to be working fine with this, so switch here too.  We
can reconsider in Xenial timeframe when we've got a later apache.

Change-Id: I05f1b1731cb6358e2e37574a90692ffe91f07ad3

modules/openstack_project/manifests/files.pp

@@ -77,6 +77,20 @@ class openstack_project::files (
     ],
   }
 
+  # Until Apache 2.4.24 the event MPM has some issues scalability
+  # bottlenecks that were seen to drop connections, especially on
+  # larger files; see
+  #  https://httpd.apache.org/docs/2.4/mod/event.html
+  #
+  # The main advantage of event MPM is for keep-alive requests which
+  # are not really a big issue on this static file server.  Therefore
+  # we switch to the threaded worker MPM as a workaround.  This can be
+  # reconsidered when the apache version running is sufficient to
+  # avoid these problems.
+
+  httpd::mod { 'mpm_event': ensure => 'absent' }
+  httpd::mod { 'mpm_worker': ensure => 'present' }
+
   ###########################################################
   # docs.openstack.org