Switch Centos/Fedora to Apache woker MPM
Switch Centos/Fedora to Apache woker MPM

In trying to debug periodic gate instability of CentOS, I noticed that
it is using the prefork mpm, while Ubuntu is defaulting to the
multi-threaded worker mpm.

One of the problems seems related to 502 proxy errors from the TLS
proxy.  We see out-of-sync timestamps in the centos TLS proxy access
logs, which might be innocent behaviour based on the prefork model or
indicate something else.

Before going too deep down this rabbit-hole, I think it is better for
consistency to use the same mpm model on all our platforms, and start
debugging from there.

Change-Id: I9881f2e7d51fdd9fc0f7fb3e37179aa53171b531

openstack-dev/devstack,noironetworks/devstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,noironetworks/devstack,group-policy/devstack,nawawi/openstack,nawawi/openstack,group-policy/devstack