QPID-4676: change External auth provider to create usernames of the form <CN>@<DC1>.<DC2>....<DCN> by default

- Allows for use of SSL Client Authentication in manner more consistent with the C++ broker
- Adds 'useFullDN' attribute to the auth provider to allow enabling use of the old behaviour


git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1463074 13f79535-47bb-0310-9956-ffa450edef68