Recognize PEP-440 prereleases in release pipeline

These days we shouldn't need to do anything special with separate
pipelines/job sets differentiated by full releases and prereleases,
but it would still be nice to be able to push prerelease versions
for folks to perform final manual checks or to exercise release
automation prior to tagging a proper release.

Change-Id: I6a45d35c242d40c4fabe6dd8aae3812327a8f743

zuul.d/pipelines.yaml

@@ -118,7 +118,7 @@
     trigger:
       gerrit:
         - event: ref-updated
-          ref: ^refs/tags/[0-9]+(\.[0-9]+)*$
+          ref: ^refs/tags/[0-9]+(\.[0-9]+)*(a|b|rc)[0-9]+$
     success:
       mysql:
     failure: