Fix raising TimeoutException in tests

TimeoutException was imported from neutron_tempest_plugin.exceptions
module but there is no such class there.
This exception is defined in tempest.lib.exceptions module and
this patch changes to use it from there properly.

TrivialFix

Change-Id: If32d8414207cdb9dd8016e8bb1f8ff47164ca1e4
2 files changed