Admission controller: user to pool resolution, authorization, and pool configs

Adds RequestPoolUtils which exposes user to pool resolution, authorization,
and relevant pool configurations by wrapping Yarn classes that provide that
functionality. (To support CDH4, those Yarn classes will come from
thirdparty/cdh4-extras.) RequestPoolUtils is created once by the backend and
the instance lives for the duration of the process.

Change-Id: I53db075555578614356d33f9d939c5378b9ec797
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1566
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins