Update deprecated use of yaml.load

Update the deprecated use of `yaml.load` without a loader, replacing it with a call to the sugar method `yaml.safe_load`. See also: https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation



Change-Id: Ia5b591349ba6da4260545335315d3d07191f76cf
Signed-off-by: Adam Israel <adam.israel@canonical.com>