rgmanager: ra2rng.xsl: parametrize and make more flexible

The output is 1:1 to the previous version, but it is now easy to change
base indentation level and internally, we use abstract blocks for tags
instead of </> kludge. Also the common optional parameters are
now enumerated outside the templates.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>