PIG-3765: Ability to disable Pig commands and operators - minor refactor

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1579863 13f79535-47bb-0310-9956-ffa450edef68
PIG-3826: Outer join with PushDownForEachFlatten generates wrong result

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1579873 13f79535-47bb-0310-9956-ffa450edef68