Add simple check that change topic is prefixed by 'spec/' and use it to
group changes as src+spec to single build. Value after prefix may be any,
but it's required to be unique for only two changes - one to OpenStack
project, and other to build spec project.
Change-Id: I7cb305d9d0b9d6715951763ce72efb06a7c878f1
Closes-Bug: #1514873