Provide a grace period for session validity

If a session has *just* expired, it may just be waiting for the
next timeout pass.  If it expired 15 seconds ago, however, it should
by all rights be beyond dead.

This matters as the calling code may be oblivious to the change of
session object under the covers if this is tripped.

Change-Id: I5be3bafb5932c287eb17c66217ba630746533310

pyghmi/ipmi/private/session.py

@@ -406,7 +406,7 @@ class Session(object):
         with util.protect(KEEPALIVE_SESSIONS):
             sess = cls.keepalive_sessions.get(session, None)
             if sess is not None and 'timeout' in sess:
-                if sess['timeout'] < _monotonic_time():
+                if sess['timeout'] < _monotonic_time() - 15:
                     # session would have timed out by now, don't use it
                     return False
         return True