Add current version information to the VTY reference manuals
Currently the only version information in the vty reference manuals is the revision history that needs to be updated manually.
This info resides in doc/manuals/*-vty-reference.xml
We now have a script to regenerate the actual vty reference (in doc/manuals/vty/*_vty_reference.xml) so updating the revision information should be automated as well.
I don't think using the revision history field in docbook makes sense for this task - at least not adding an entry for every change/commit. Maybe we just want to use one revision entry and update that to the date of the commit and use the git describe string of the commit as the revnumber.
That way it is at least clear which version this documentation belongs to.