[SPARK-2687] [yarn]amClient should remove ContainerRequest

in https://issues.apache.org/jira/browse/YARN-1902, after receving allocated containers,if amClient donot remove ContainerRequest,RM will continually allocate container for spark AppMaster.@tgravescs
so even though the preferred node locality stuff isn't working right now I think I would rather see us attempt to do the right thing here.    If there weren't any preferred nodes specified then taking any of the requests works just fine, but if there were host or rack specific requests we should remove the ones that were actually allocated to us.