Clean up in requirements job

We don't run this job with a fully clean workspace (git clean in the
requirements repo does not seem to also delete git repositories that
have been cloned as subdirs) So we need to remove and re-clone.

Also, while looking in to this, concurrent: false was missing. I'm
pretty sure we don't want to run more than one of these.

Change-Id: I09d1d07b815c348df5d9ae664b84dba9d5d169f0

modules/jenkins/files/slave_scripts/propose_requirements_update.sh

@@ -42,6 +42,7 @@ EOF
         set -e
     fi
 
+    rm -rf $(basename $PROJECT)
     git clone --depth=1 ssh://$USERNAME@review.openstack.org:29418/$PROJECT.git
 
     PROJECT_DIR=$(basename $PROJECT)

modules/openstack_project/files/jenkins_job_builder/config/requirements.yaml

@@ -26,6 +26,7 @@
 - job:
     name: propose-requirements-updates
     node: proposal
+    concurrent: false
 
     builders:
       - gerrit-git-prep