Support legacy clients for config_file_sync

During a rolling pool upgrade, if a slave xapi restarts then it will attempt to
sync config files, and will expect the old-style passwd file response. The
config_file_sync handler now supports versioned URIs, so that clients accessing
the original URI will receive the legacy config files, while clients accessing
the URI with a version number appended will receive the new config files.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>