ERP5: add xmlsec
This includes xmlsec, the C library and its python bindings in ERP5, using the latest versions that are compatible with python 2. `1.3.9` is the latest version of the python bindings with python 2 support and `1.2.30` is the latest version of the C library compiling with this version of the python bindings. See merge request nexedi/slapos!1272