recreate the http session for each scenario

reason: the http session would be the same for all tests in
features/daily. When running the tests in alphabetical order, as
lettuce does, infos.feature uses xivo-confd very early, creating the
http session and backup.feature restarts xivo-confd a short while
after that, which breaks the stored session until it is reset by
confd/devices.feature when setting HTTP_PROXY.