[train-only] Make sure UpgradeLevelNovaCompute is empty string for upgrade

This value causes issues with live migration that will be
addressed later (see related launchpad bug).
    
This patch assigns the empty string value to the parameter
to make sure it doesn't have any other value assigned before
performing an upgrade.
    
Co-Authored-By: Jose Luis Franco Arza <jfrancoa@redhat.com>
Closes-Bug: rhbz#1849235
Related-Bug: rhbz#1851239
Related-Bug: #1871482

Change-Id: If5f8c3391efa9d283bcbbde388f6e1b8143f2db5

environments/lifecycle/upgrade-converge.yaml

@@ -3,7 +3,6 @@
 # by the operator. Please use additional -e env.yaml.
 parameter_defaults:
   EnablePackageInstall: false
-  UpgradeLevelNovaCompute: ''
   UpgradeInitCommonCommand: ''
   UpgradeInitCommand: ''
   # FFU params

environments/lifecycle/upgrade-prepare.yaml

@@ -9,7 +9,7 @@ resource_registry:
 
 parameter_defaults:
   EnablePackageInstall: true
-  UpgradeLevelNovaCompute: auto
+  UpgradeLevelNovaCompute: ''
   UpgradeInitCommonCommand: |
     #!/bin/bash
     set -eu