Ignore 'use_tpool' option
Ignore 'use_tpool' option

The sheer act of registering the 'use_tpool' option seems to be enough
to cause the following warning:

   NotSupportedWarning: Configuration option(s) ['use_tpool'] not
   supported

Add it to the list of ignored opts, thus silencing this warning.

Change-Id: I4a5f8958c3e0fc68fa3a4ae979f284c4fbd9b82b

openstack/oslo.db,openstack/oslo.db