Finish adding configurable log rotation

* Update log backupCount name and description.

* Add additional error handling to --log-backups

* test --log-backups flag

* Pass log_backups value to RotatingFileHandler

* Test that log_backups is properly used

* add _test_success_common

* Add test_success_with_rollover

* Add test_success_without_rollover

* mock stderr in cli tests

* Set log_backups in PostArgParseSetupTest

* Rename "log backups" to "max log backups"

certbot/cli.py

@@ -850,15 +850,11 @@ def prepare_and_parse_args(plugins, args, detect_defaults=False):  # pylint: dis
         None, "-t", "--text", dest="text_mode", action="store_true",
         help=argparse.SUPPRESS)
     helpful.add(
-        # Note, 1 is subtracted from max_log_count in certbot/main.py
-        #> to correct an off by one error.
-        [None, "paths"],
-        "--max-log-count", dest="max_log_count",
-        type=int, default=1000,
-        help="Specifies the maximum number of certbot log files "
-               "that will be kept. 0 disables log rotation. 1 causes "
-               "only the log from the most recent run to be kept. "
-               "2+ enables log rotation at start of certbot execution.")
+        None, "--max-log-backups", type=nonnegative_int, default=1000,
+        help="Specifies the maximum number of backup logs that should "
+             "be kept by Certbot's built in log rotation. Setting this "
+             "flag to 0 disables log rotation entirely, causing "
+             "Certbot to always append to the same log file.")
     helpful.add(
         [None, "automation", "run", "certonly"], "-n", "--non-interactive", "--noninteractive",
         dest="noninteractive_mode", action="store_true",
@@ -1385,3 +1381,27 @@ class _RenewHookAction(argparse.Action):
             raise argparse.ArgumentError(
                 self, "conflicts with --deploy-hook value")
         namespace.renew_hook = values
+
+
+def nonnegative_int(value):
+    """Converts value to an int and checks that it is not negative.
+
+    This function should used as the type parameter for argparse
+    arguments.
+
+    :param str value: value provided on the command line
+
+    :returns: integer representation of value
+    :rtype: int
+
+    :raises argparse.ArgumentTypeError: if value isn't a non-negative integer
+
+    """
+    try:
+        int_value = int(value)
+    except ValueError:
+        raise argparse.ArgumentTypeError("value must be an integer")
+
+    if int_value < 0:
+        raise argparse.ArgumentTypeError("value must be non-negative")
+    return int_value