Validate configuration objects more thoroughly when parsing

[shaynevanasperen]

Currently, we perform some rudimentary validation of configuration objects when reading files from disk. We'd like to perform this validation more thoroughly, so that any potential errors are found before we begin the workflow to synchronize the configuration to Kong. This will help to make sure that the workflow doesn't fail halfway through and leave Kong in a broken/undefined state.

Things we'd like to validate are:

• Properties of Service are set to valid values
• Properties of Route are set to valid values
• Properties of Plugin are set to valid values
• Only plugins which are installed are valid for use (the list of installed plugins can be queried from Kong)
• Plugin configuration is valid according to that plugin's schema (which we can query from Kong)