QPID-4696: Group broker attributes on editing UI, improve attribute descriptions, add attributes units.
           Remove obsolete GroupPrincipalAccessor.
           Fix flow control attribute validation.
           Fix constant name for default group provider attribute.
           Remove redundant comments.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1464115 13f79535-47bb-0310-9956-ffa450edef68