feature(worker): implemented mirror role (master/slave) option

tests/worker.conf

@@ -29,6 +29,7 @@ command = "/tmp/tunasync/bin/myrsync2.sh"
 upstream = "https://aosp.google.com/"
 interval = 2
 mirror_dir = "/tmp/tunasync/git/AOSP"
+role = "slave"
 	[mirrors.env]
 	REPO = "/usr/local/bin/aosp-repo"