Revert the incomplete rename of _get_answer_on_exten_status_fn

This reverts commit c2ea2bbcdbe5c51ce0494787dd847c82b1e27cfa.
add an env variable SNOM_SB_ANSWER_DELAY

To make troubleshooting easier the SNOM_SB_ANSWER_DELAY can be used to
change the default delay before a snom switchboard tries to answer a call.