Bug 8746: Fix indexation in DOM index mode

Bug 8746: Fix indexation in DOM index mode

When in DOM index mode, files exported by `rebuild_zebra.pl -x` are
wrapped by '<collection></collection>' tag.
This is a problem because splitting files produces invalid files.
This is fixed by adding the missing <collection> tags in each generated
file.
Another problem was that the wrong zebra configuration file was used.
The script now uses C4::Context->zebraconfig($server)->{config} to know
which configuration file has to be used.

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>