[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
When you request containers from yarn and specify hosts or racks then it is not guaranteed that you get them back in the order you requests them.  Certain hosts may be busy, not available, etc.  You might get a rack request fullfilled instead of a host request, etc.  Since the preferred locality stuff is broken right now anyway (SPARK-2089), we can fix this up separately if you would prefer?