Update to use safe_load

The use of a bare yaml.load with no Loader is deprecated. Use the sugar shortcut safe_load per https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation



Change-Id: I856d4263fcf20453157f9d3835289e665d1664bf
Signed-off-by: Adam Israel <adam.israel@canonical.com>