Admission controller: use request memory estimate in admission

Change-Id: I86ba26df434e9297b11abe349ea237fea9b04b87
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1622
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins