Disable flaky explain tests due to inconsistent per-host mem requirements

Change-Id: Ie372696c4986dc7f7c8f7fc074c41b89bd65f456
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1939
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins
Minor test cleanup

* Prefer 'refresh <table name>' over 'invalidate metadata'
* Remove the 'RELOAD' test setup option that was used by only 1 test.
* Delete a .py test file that seems to be a duplicate

Change-Id: I890546635840bb8f4d55789a89f8c8f33e40d001
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1933
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins
Admission controller: Fix default parameter to QueuePlacementPolicy.assignAppToQueue()

We should be passing YarnConfiguration.DEFAULT_QUEUE_NAME as the default value to
QueuePlacementPolicy.assignAppToQueue(). This caused a problem when the placement
policy was Specified(create=true) where the returned name was "root." because it
literally created a pool with the name "" below root.

Change-Id: I41cc2848d78bf8bd308292e80d6f6c7d06437ae9
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1942
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins