Provide methods to check node attributes

[111pontes]

YANG nodes (e.g. containers, leaves, etc) may have a number of attributes (e.g. config, status, default, etc). While those are described in the API documentation, some users will need methods to check these attributes programmatically. In YDK 0.5.1 and earlier, the `is_config()' method provided a mechanism to check whether certain nodes represented a configuration node or not. A more holistic approach should be made available that would cover all attributes and all node types.