Remove puppet-unit 2.6 gate for puppet-tempest

Puppet 2.6 unit tests were removed as they aren't supported anymore.

Change-Id: I417875be33887d9578be33cadc67fb8de0420de0
Reviewed-on: https://review.openstack.org/30909
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins