Modify ipa build job parameters

Add IPA_BRANCH parameters as IPA_REPO_REF should actually
contain the commit hash that specifies the IPA version on
the brnach specified by IPA_BRANCH variable.

Change-Id: I1496cd4c2d5be5508b7f37ffeab1bc98005310d0
diff --git a/jjb/airship/job_ipa_image_building.yml b/jjb/airship/job_ipa_image_building.yml
index d2752e8..70df57d 100644
--- a/jjb/airship/job_ipa_image_building.yml
+++ b/jjb/airship/job_ipa_image_building.yml
@@ -39,8 +39,12 @@
         description: 'Gerrit refspec of the patch we want to test. Example: refs/changes/84/800084/22'
     - string:
         name: IPA_REPO_REF
+        default: 'HEAD'
+        description: 'Ironic Python Agent repo commit hash to build'
+    - string:
+        name: IPA_BRANCH
         default: 'master'
-        description: 'Ironic Python Agent code branch'
+        description: 'Ironic Python Agent repo branch to build'
     - string:
         name: STAGING
         default: 'true'