Prevent abnormal timeout values from breaking sync with ironic

I had a case when an operator set the timeout too a abnormally large
value, which caused OverflowError in the timeout clean_up task, which,
in turn, prevented ironic-inspector from ever running node list sync
with ironic.

This change makes two corrections:
* The timeout configuration is limited to 10 years (0 can still be used
  to disable the timeout completely).
* Errors in the clean_up task do not prevent the node list sync from
  running.

Change-Id: Ie3ad29a4abb9ac58c41b776042f80dff6a9c72d2
Story: #2004807
Task: #28963

ironic_inspector/conductor/manager.py

@@ -133,10 +133,14 @@ def periodic_clean_up():  # pragma: no cover
     try:
         if node_cache.clean_up():
             pxe_filter.driver().sync(ir_utils.get_client())
-        sync_with_ironic()
     except Exception:
         LOG.exception('Periodic clean up of node cache failed')
 
+    try:
+        sync_with_ironic()
+    except Exception:
+        LOG.exception('Periodic sync of node list with ironic failed')
+
 
 def sync_with_ironic():
     ironic = ir_utils.get_client()

ironic_inspector/conf/default.py

@@ -41,6 +41,9 @@ _OPTS = [
                       'options. "noauth" will disable all authentication.')),
     cfg.IntOpt('timeout',
                default=3600,
+               # We're using timedelta which can overflow if somebody sets this
+               # too high, so limit to a sane value of 10 years.
+               max=315576000,
                help=_('Timeout after which introspection is considered '
                       'failed, set to 0 to disable.')),
     cfg.IntOpt('clean_up_period',