Otherwise can cause issues when enabling prefork as mpm_event is enabled
by default in httpd. A recent change in puppet-apache [1] has added
httpd.modules.conf directory to load, which includes default mpm config
that conflicts with prefork.
In the past we disabled purge because of issues with nagios [2], however
nagios deployment is not longer supported by packstack, so we can safely
enable apache config purge which is the default option.
[1] 6425cc46a8
[2] 686625e17b
Change-Id: Ifdccd8609658b3170b8356bddc5062f252677d40
08a67720db