Fix yaml.load() deprecation warnings by using SafeLoader, e.g.:

/home/barryprice/juju-deployer/deployer/tests/test_config.py:102: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.

Reviewed-on: https://code.launchpad.net/~barryprice/juju-deployer/resolve-yaml-warnings/+merge/391925
Reviewed-by: Benjamin Allot <benjamin.allot@canonical.com>