HBASE-8372 Provide mutability to CompoundConfiguration
This patch also addresses:
HBASE-8347 TestSecureLoadInc* tests hang repeatedly getting UnsupportedOperationException in hadoop2 profile


git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1471118 13f79535-47bb-0310-9956-ffa450edef68