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

Based on patch from Jakub Scholz, with some updates from me.

merged from trunk r1463074


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1463192 13f79535-47bb-0310-9956-ffa450edef68