cli: Fix RetryingCaucaseClient.

Python was unexpectedly binding utils.until to this class, causing "self"
argument to be automatically generated, which is not expected by this
function.
Tests do not exercise this code path because they are overriding this
property, precisely to check that it gets called...
Also, add docstring.

caucase/cli.py

@@ -40,7 +40,14 @@ from .client import (
 )
 
 class RetryingCaucaseClient(CaucaseClient):
-  _until = utils.until
+  """
+  Similar to CaucaseClient, but retries indefinitely on http & socket errors.
+  To use in long-lived processes where server may not be available yet, or is
+  (hopefuly) temporarily unavailable, etc.
+
+  Retries every 10 seconds.
+  """
+  _until = staticmethod(utils.until)
 
   def _request(self, connection, method, url, body=None, headers=None):
     while True: