qa/standalone/test_ceph_daemon: Fix hang when CEPH_DAEMON is not set

qa/standalone/test_ceph_daemon: Fix hang when CEPH_DAEMON is not set

When running test_ceph_daemon.sh from the root dir and not setting
$CEPH_DAEMON manually, the call hangs at:

$ ./qa/standalone/test_ceph_daemon.sh
[...]
+ for p in $PYTHONS
+ echo '=== re-running with python3 ==='
=== re-running with python3 ===
++ which python3
+ ln -s /usr/bin/python3 /tmp/tmp.6hneCsNMio/python
+ echo '#!/tmp/tmp.6hneCsNMio/python'
+ cat

Check that there is a ceph-daemon found before continue.

Signed-off-by: Thomas Bechtold <tbechtold@suse.com>