Based on a discussion with Daniel Beck, implemented a master kill switch as an...

Based on a discussion with Daniel Beck, implemented a master kill switch as an admin configurable feature.

The wiki page already talks about how an admin can do this: https://wiki.jenkins-ci.org/display/JENKINS/Slave+To+Master+Access+Control#SlaveToMasterAccessControl-Disabletheaccesscontrolmechanismaltogether

There's a risk in making this too easy, but we keep finding more plugins that need SECURITY-144 adjustments, and I'm really worried
about SECURITY-144 fix blowing up in the face of too many people.