Mark OpenStack stable branches as non-voting for pecan; they're too unreliable.

Pecan is very regularly held up by bugs that haven't been backported to stable
branches.  While checks are still useful to ensure that pecan hasn't
legitimately broken something, the majority of the failures I see are
unrelated issues in the projects themselves (requiring time-consuming
backports).

Change-Id: I0dcab730bd637dc33369f74ba3eb363314025469
Merge "Mark OpenStack stable branches as non-voting for pecan; they're too unreliable."