Ensure CrateDB process gets terminated on cr8 sigterm

The CrateDB process leaked if `cr8 run-crate` was killed via a SIGTERM.
For example, in a bash script like:

    cr8 run-crate latest-nightly &
    N1=$!
    while ! crash --hosts localhost:4200 -c "select 1" > /dev/null 2>&1; do
        sleep 1
    done
    jps
    kill $N1
    jps

cr8/run_crate.py

@@ -18,12 +18,13 @@
 import socket
 import ssl
 import platform
+import signal
 from datetime import datetime
 from hashlib import sha1
 from pathlib import Path
 from functools import partial
 from itertools import cycle
-from typing import Optional, Dict, Any, List, NamedTuple
+from typing import Dict, Any, List, NamedTuple
 from urllib.request import urlopen
 
 from cr8.java_magic import find_java_home
@@ -790,6 +791,12 @@ def run_crate(
             keep_data,
             java_magic=not disable_java_magic,
     ) as n:
+
+        def stop(signum, frame):
+            n.stop()
+            sys.exit(0)
+
+        signal.signal(signal.SIGTERM, stop)
         try:
             n.start()
             n.process.wait()