Fix namespace in generated WSDL

The generated WSDL contains soap:operation nodes inside wsdl:operation
nodes, but the namespace used is incorrect, which leads to the WSDL not
being valid.

This is the fix for https://bugs.launchpad.net/wsme/+bug/1235238

Signed-off-by: Xavier Trochu <xavier.trochu@edpsciences.org>