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.
Merge pull request #1972 from jimis/always_kill_running_servers

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