RHEL/Update: replace wc by yum to check updates

Similar to I8bd89f2b24bafc6c991382b9eb484cfa9a2f8968,
use yum to check if updates are available during RHEL registration and
run upgrades only when there are some.

Change-Id: I42cd699d21cbec7d754edf1b8d83e75de0f2c7d9

extraconfig/pre_deploy/rhel-registration/rhel-registration.yaml

@@ -173,11 +173,12 @@ resources:
       config: |
         #!/bin/bash
         set -x
-        num_updates=$(yum list -q updates | wc -l)
-        if [ "$num_updates" -eq "0" ]; then
-           echo "No packages require updating"
-           exit 0
-        fi
+        # yum check-update exits 100 if updates are available
+        set +e
+        check_update=$(yum check-update 2>&1)
+        check_update_exit=$?
+        set -e
+        if [[ "$check_update_exit" == "100" ]]; then
             full_command="yum -q -y update"
             echo "Running: $full_command"
             result=$($full_command)
@@ -185,6 +186,13 @@ resources:
             echo "$result"
             echo "yum return code: $return_code"
             exit $return_code
+        elif [[ "$check_update_exit" == "1" ]]; then
+            echo "Failed to check for package updates"
+            echo "$check_update"
+        else
+           echo "No packages require updating"
+           exit 0
+        fi
 
   UpdateDeploymentAfterRHELRegistration:
     type: OS::Heat::SoftwareDeployment