move single-request-reopen from ks.cfg

[juliandunn]

PR#165 added single-request-reopen to the CentOS /etc/resolv.conf. However before I merged this I didn't realize that it's only supported on CentOS 6.x; therefore it should be moved into something a little more specific to the distro (postinstall script?)

[rjocoleman]

@juliandunn my bad, I'll create a post install script.

[rjocoleman]

I've written a postinstall script that should work for Centos 6, RHEL6, Fedora 19 and Fedora 20.
As best I can tell this effects virtualbox only, so I've limited the post-install script to the virtualbox builds.

I'm just downloading all those boxes (expect RHEL) to reproduce the original issue and verify they need this. Then I'll build the boxes with the fix to ensure it works.. then after all of those downloads and builds I'll send a PR!