Zuul: Use underscores for ansible vars

This tweaks the github mirroring job to use underscores
instead of hyphens in its secret name.  Although the job
seems to be running just fine, hyphens have caused issues
in ansible variables in other contexts, and this change
enforces consistency.

Change-Id: I4b8c644d434cf5878d437b5931dfa5ad6315b71b

.zuul.yaml

@@ -36,7 +36,7 @@
       git_mirror_repository: airshipit/images
     secrets:
       - name: git_mirror_credentials
-        secret: images-airshipit-github-secret
+        secret: images_airshipit_github_secret
         pass-to-parent: true
 
 - job:
@@ -57,7 +57,7 @@
         label: ubuntu-bionic
 
 - secret:
-    name: images-airshipit-github-secret
+    name: images_airshipit_github_secret
     data:
       user: git
       host: github.com