Re-init "model/classes/system/.git" with the relative path

Jenkins pipeline makes an absolute path in the .git file of
the submodule:

    # cat model/classes/system/.git
    gitdir: /home/jenkins/workspace/workspace/swarm-create-cfg-config-drive/model/.git/modules/classes/system

- remove the file "model/classes/system/.git" after clonning
  the model and re-init the submodule to get the relative path:

    # cat model/classes/system/.git
    gitdir: ../../.git/modules/classes/system

Closes-Bug: #PROD-27006
Change-Id: Ia1a568ba158f1eb4d9544d293c817d7d95f4f10c
diff --git a/jobs/pipelines/swarm-create-cfg-config-drive.groovy b/jobs/pipelines/swarm-create-cfg-config-drive.groovy
index 46d5010..a734235 100644
--- a/jobs/pipelines/swarm-create-cfg-config-drive.groovy
+++ b/jobs/pipelines/swarm-create-cfg-config-drive.groovy
@@ -145,8 +145,9 @@
             sh "mkdir model && cd model && tar zxfv ../model.tar.gz"
         } else {
             sh "git clone --recursive $model_url -b ${MCP_VERSION} model"
-            // remove .git file with hardcoded path
+            // remove .git file with absolute path and re-init the file with relative path
             sh "rm model/classes/system/.git"
+            sh "cd model && git submodule update"
         }
     }