NodeManager blocks kill attempts during startup if ResourceManager is down

Found while testing a 2.7.3 RC:

• If you start the NodeManager and not the RM, the NM has a handler for SIGTERM and SIGINT that blocked my Ctrl-C and kill attempts during startup. I had to kill -9 it.

stevel@apache.org tells me that this is potentially addressed by YARN-679, though that's a large patch.