Tie all verify/merge/build to opnfv-build-ubuntu

Tie all verify/merge/build to opnfv-build-ubuntu

We reconfigured centos build server now and it is important to
know which jobs require centos and which ones ubuntu. The machines
with ubuntu were labelled with opnfv-build previously, preventing
us from keeping track of which projects require what OS. This tries
to solve that.

Change-Id: I1fb2912ec49f5bc2781853e500508d9992d59fbb
Signed-off-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>