Alternative implementation of the policy document parser

[narrieta]

Discussed with @mgunnala an alternative approach to write the parse for policy documents. It is similar in concept to a recursive descendent parser and produces code that is simpler to write, read, and debug.

We also decided to make the document version a required attribute.

Lastly, I added code to also parse the runtimePolicy for extensions.

[narrieta]

@mgunnala - FYI: as part of my latest changes, I changed ExtensionPolicyEngine.should_allow_extension() and ExtensionPolicyEngine.should_enforce_signature_validation() to take an extension name, rather than an Extension object. This is in order to avoid the implicit dependency on azurelinuxagent.common.protocol.restapi; there is no need to depend on that module.