Change I5f5cec5a1622be4983b4d0db392453d1c489fa40 allowed the
net_config_override file to be yaml as well as json, but if
enable_validations is true the _validate_no_ip_change assumes the
network config is json.
This change loads this file as yaml in _validate_no_ip_change.
Change-Id: Iaeadf412865c059d252ffc7087db3853569b74ee
Related-Bug: #1927090