Reference jenkins master in workers.

The "stop:" function should be qualified by the name of the jenkins master
to make it globally unique (eg "stop:jenkins.example.com").

Supply the name of the master in the WORK_DATA that is sent to the client
when the job starts building so that the client can direct a subsequent
"stop:" request to the right worker.

Change-Id: I0112b84ae614ce4faaed880ea3d1073674dfe5fe