Remove unnecessary NodePoolBuilder thread

Originally, the thread started with the NodePoolBuilder.runForever()
method did all the work of the builder. This functionality is moving
to inside the BuildScheduler. As a result, the thread used for
NodePoolBuilder is no longer necessary. The intention is to (eventually)
totally decouple these classes so that NodePoolBuilder is used only to
start and stop the builder thread of execution. It's run() method is
renamed to start() to better reflect its purpose.

Change-Id: Ief00e4a06bd919e99dbff07f7238ac51035b06ef

nodepool/cmd/builder.py

@@ -16,7 +16,6 @@ import argparse
 import extras
 import signal
 import sys
-import threading
 
 import daemon
 
@@ -62,9 +61,7 @@ class NodePoolBuilderApp(nodepool.cmd.NodepoolApp):
             self.args.upload_workers)
 
         signal.signal(signal.SIGINT, self.sigint_handler)
-
-        nb_thread = threading.Thread(target=self.nb.run)
-        nb_thread.start()
+        self.nb.start()
 
         while True:
             signal.pause()