OpenShift GSS-TSIG DNS Plugin (Active Directory & IPA)

Based on the nsupdate plugin (ruby193) I've created a new separate GSS-TSIG specific plugin that I've tested and works (RHEL 6 + OSE with Windows 2008 R2 x64 AD as well as RHEL 6 IPA/IdM).

There are a few caveats:
* when testing against OpenShift Enterprise I had to ensure that the gem was installed manually (gem install --local /path/to/gem)
* the sources need to be manually placed into the rpmbuild/SOURCES directory as they don't currently exist within the OpenShift repo.

Rebased & squashed https://github.com/openshift/origin-server/pull/1550