Add max limit to agent_down_time

The agent_down_time ends up being passed to an eventlet green-thread;
under the hood, this uses a CPython C-types interface with a limitation
of (2^32 / 2 - 1) INT_MAX (as defined in C) where int is usually 32 bits

I have set the max value to (2^32 / 2 - 1)/1000 as agent_down_time
configured in seconds, this ends up being 2147483.

This patch is required as passing a larger number
causes this error: OverflowError: timeout is too large

If a user currently has a value larger than (2^32 / 2 - 1)/1000 set,
Neutron Server will fail to start and will print out a very helpful
error message.

Closes-Bug: #2028724
Change-Id: Ib5b943344cddbd468c00768461ba1ee00a2b4c58

neutron/conf/agent/database/agents_db.py

@@ -16,7 +16,12 @@ from neutron._i18n import _
 from neutron.common import _constants
 
 AGENT_OPTS = [
+    # The agent_down_time value can only be a max of INT_MAX (as defined in C),
+    # where int is usually 32 bits. The agent_down_time will be passed to
+    # eventlet in milliseconds and any number higher will produce an OverFlow
+    # error. More details here: https://bugs.launchpad.net/neutron/+bug/2028724
     cfg.IntOpt('agent_down_time', default=75,
+               max=((2**32 / 2 - 1) // 1000),
                help=_("Seconds to regard the agent as down; should be at "
                       "least twice report_interval, to be sure the "
                       "agent is down for good.")),

releasenotes/notes/agent_down_time_max-af3b62763aaa2fe5.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    The config option ``agent_down_time`` is now limited to a maximum value of
+    `2147483`, as neutron-server will fail to start if it is configured higher.
+    See bug `2028724 <https://bugs.launchpad.net/neutron/+bug/2028724>`_ for more information.