cli: Use utcnow() as everywhere else

Timedeltas between utcnow() and now() depend on system timezone, so
they can last many hours when a few seconds is intended.

The now() here was entered by mistake.

/reviewed-on nexedi/caucase!5

caucase/cli.py

@@ -81,7 +81,7 @@ class RetryingCaucaseClient(CaucaseClient):
           ),
           file=self._log_file,
         )
-        self._until(datetime.datetime.now() + datetime.timedelta(0, 10))
+        self._until(datetime.datetime.utcnow() + datetime.timedelta(0, 10))
 
 _cryptography_backend = default_backend()