BibCatalog: add default RT user

• Adds default user account information to the BibCatalog RT system which will be used when no Invenio user id is given on ticket generation. Useful for future processes where the system will generate RT tickets on behalf of the system. (fixes #735)

• An Invenio user connected to an RT account will still be required when searching for, changing and retrieving tickets.

• BibCatalog configuration updates in invenio.conf reflects these changes.