Enable requirement update proposal jobs

Apparently we needed to use the magical ZUUL_REF job variable to get the
branch info from a change in the Zuul post pipeline. Use it and only
propose requirements updates to the branches that were updated.

Closes-bug: 1246249
Change-Id: I7adfd19e502846d3a07cde1808fdc79164c061a9
Merge "Enable requirement update proposal jobs"