Recover from commands executed with unknown credentials

We'll fall back to executing the command without authentication if the
command fails in a very specific way. This is a sign the provided
private key is unknown to the Jenkins master. This exception is commonly
thrown the first time a Chef run enables authentication on the Jenkins
master. This should also fix some cases of JENKINS-22346.
Update README for recent auth changes
Merge pull request #289 from opscode-cookbooks/schisamo/executor-with-unknown-creds

Recover from commands executed with unknown credentials