ReadFileOnMaster etc. did not actually work

Attempted code to make non-direct private key sources accessible from slaves did not actually work: the call to `Hudson.getInstance()` was null in this context.

Anyway it is simpler and more efficient to just replace the source with a direct entry before serializing the credentials to send to the slave.

(Bug possibly concealed by otherwise superfluous remoting call from `CliGitAPIImpl.createSshKeyFile`.)