gp_parse: Add a generalize XML function to the top level parser

gp_parse: Add a generalize XML function to the top level parser

In this function we take XML and using the required metadata, we rewrite
it into a generic form using entities. ElementTree unfortunately does
not allow us to store unescaped entities, and so we must do a textual
replace on the output XML.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>