Always kill cf-serverd in the end of relevant acceptance tests

There were stale cf-serverd processes on the buildslaves (for whatever
reason), and any subsequent run was not killing them, but obviously
failing because server was failing to start (had already bound the
port). Better to always try to kill stale cf-serverd processes.

(cherry picked from commit b091e405ec7095191ac1313ca8d8e1f43b29f3a1)