Change all calls to YAML.load into YAML.safe_load

Now, when all serialization of YAML data is ensured to be Data, we must also ensure that no unsafe data can be loaded using YAML. Psych provides the method YAML.safe_load to accomplish this.

We do have some places were we still load objects that are not Data for backward compatibility causes. We allow Symbol keys in hiera in some places and we provide a YAML-specificit tag for Puppet::Node::Facts to make it directly deserializable into instances of that class. Such exceptions can (and should) be declared specifically as arguments to YAML.safe_load.

Also need to review JSON.load