QPID-4863: Validate plugin attribute changes and throw UnsupportedOperationException where attribute changes are not supported

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1483861 13f79535-47bb-0310-9956-ffa450edef68
QPID-4739: Remove redundant validation of keystore attributes as they are validated at keystore creation

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1483862 13f79535-47bb-0310-9956-ffa450edef68