jobs/coverity.yml: add parameter for git branch

Allow configuring a different git branch before starting the build. This
is especially useful to reproduce a failure that only happens when using
coverity tools during a build.

Change-Id: I5f7c615ff5af4bf1f34ccef62767b5ca71cd2d8d

jobs/coverity.yml

@@ -24,13 +24,18 @@
       - build-discarder:
           days-to-keep: 30
           num-to-keep: 30
+    parameters:
+      - string:
+          name: BRANCH
+          description: osmo-ci.git branch
+          default: 'origin/master'
     scm:
       - git:
           url: git://git.osmocom.org/osmo-ci
           git-config-name: 'Jenkins Builder'
           git-config-email: 'jenkins@osmocom.org'
           branches:
-            - 'origin/master'
+            - '$BRANCH'
     triggers:
       - timed: "@daily"
     publishers: