tests: ceph-helpers.sh reduce kill_daemon verbosity

tests: ceph-helpers.sh reduce kill_daemon verbosity

When a test fails, the script returns immediately and kill_daemon
function is called to cleanup. It is quite verbose and requires
scrolling hundreds of lines back to find the actual error
message. Turn off the shell trace to reduce the verbosity and improve
error output readability.

The kill_daemon cannot just turn off set -x because it may be called by
a test, not just at the end of the run. Instead the kill_daemon function
checks if tracing is activated and temporarily disables it.

Also get rid of the find standard error that commonly happens when
kill_daemon is called to verify there are no leftovers and the test
directory does not exist.

Signed-off-by: Loic Dachary <ldachary@redhat.com>