Update install_osm.sh to properly checkout DEVOPS_GERRIT_REFSPEC

Change-Id: Ida60cc8eb82222a559ea63775cb848ddd5e2de16
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/installers/install_osm.sh b/installers/install_osm.sh
index 03559a7..e654d6b 100755
--- a/installers/install_osm.sh
+++ b/installers/install_osm.sh
@@ -91,5 +91,6 @@
 sudo DEBIAN_FRONTEND=noninteractive apt-get -y install git
 DEVOPS_PATH=$(mktemp -d /tmp/osm-devops-XXXXXX)
 git clone https://osm.etsi.org/gerrit/osm/devops.git $DEVOPS_PATH
-git -C $DEVOPS_PATH checkout $DEVOPS_GERRIT_REFSPEC
+git -C $DEVOPS_PATH fetch "https://osm.etsi.org/gerrit/osm/devops" $DEVOPS_GERRIT_REFSPEC
+git -C $DEVOPS_PATH checkout FETCH_HEAD
 $DEVOPS_PATH/installers/full_install_osm.sh -t $DOCKER_TAG "$@"