Add missing 'solum-specs' section in zuul layout

Zuul was not picking our jobs after a commit or even a recheck on
solum-specs on gerrit.

solum-specs sections was missing in zuul config

Change-Id: I0db1641ab2396b11e9cd4fb57631e73e4298acfc
Merge "Add missing 'solum-specs' section in zuul layout"