slaves file not present on yarn-master

Attempting to use /usr/lib/hadoop/sbin/[stop|start]-yarn.sh doesn't restart the nodemanagers. This is because we do not have the /etc/hadoop/conf/slaves file on a standalone yarn master. We need to sync this file from hdfs-master to yarn-master on every update.

Alternatively, we can construct the slaves file on the yarn master just like we do for the hdfs master. This is probably a better route since it's possible someone would have a datanode without a nodemanager (which would be an hdfs slave; not a yarn slave), and conversely, a nodemanager without a datanode (which would be a yarn slave; not an hdfs slave).