Make nova-clear-rabbit-queues conditional in openstack-nova.spec

Current nova spec file expects the file "nova-clear-rabbit-queues"
in the bindir. Until icehouse, nova was generating that file.
But the file is gone from Juno onwards
https://github.com/openstack/nova/blob/master/setup.cfg
This commit fixes the openstack-nova.spec file to include
nova-clear-rabbit-queues only if the tag is newer than 2014.1

Closes-Bug: #1326974
Change-Id: I9fc9e71aa3cd48f2477b225b93365740c5b8bc88