Add an admin option to use NSS instead of OpenSSL

This should solve a range of issues with bad SSL certificates, as observed with la banque poste or la caisse d'épargne.

This code snippet should do, given a path to NSS:

from weboob.browser.nss import init_nss, inject_in_urllib3, create_cert_db, certificate_db_filename
path = weboob_section.get('nss_dir')
if not os.path.exists(path):
    os.makedirs(path)
if not os.path.exists(os.path.join(path, certificate_db_filename())):
    create_cert_db(path)
init_nss(path)
inject_in_urllib3()

This would also require that libnss and python-nss are installed on the server.