Upgrading to RHV 4.0 (hosted engine) fails with 'Configuration value not found:' (key=conf_image_UUID)

Issue

• When trying to upgrade to RHV4.0 (hosted engine), it fails with the following error:

DEBUG otopi.context context._executeMethod:142 method exception
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/otopi/context.py", line 132, in _executeMethod
    method['method']()
  File "/usr/share/ovirt-hosted-engine-setup/scripts/../plugins/gr-he-upgradeappliance/core/misc.py", line 67, in _init
    self._config.get(config.ENGINE, config.CONF_IMAGE_UUID),
  File "/usr/lib/python2.7/site-packages/ovirt_hosted_engine_ha/env/config.py", line 154, in get
    key
KeyError: 'Configuration value not found: file=/etc/ovirt-hosted-engine/hosted-engine.conf, key=conf_image_UUID'
ERROR otopi.context context._executeMethod:151 Failed to execute stage 'Initializing': 'Configuration value not found: file=/etc/ovirt-hosted-engine/hosted-engine.conf, key=conf_image_UUID'