Removed rhel6 jobs from Murano projects config

Change-Id: I7bd1bb866452d18fccd96485a3fb640a828c3ecf
Reviewed-on: https://review.openstack.org/29758
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: matthew wagoner <zxkuqyb@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins