Don't log exception for None timeout_mins

If a request contains a timeout_mins key but it is None (which is
now the case for all requests sent via python-heatclient), we log
an exception, but really we just want to ignore the value and
continue (as well as other "None" values, ie '0' which was the
previous default for no key)

So split the test for a key and conversion to int, and only log
the exception if there is an error doing the type conversion,
not if we get a value which implies ignoring/doing nothing.

Change-Id: I6b8d1f053b47fd4c22a5df636481a95bd046b1df
Closes-Bug: #1297759
Merge "Don't log exception for None timeout_mins"