Refactored parsing of YANG uses statement.

Uses statement now add nodes from referenced grouping to context where uses is defined. Added GroupingMember interface as marker interface for nodes defined in grouping.
Fixed parsing of config statement: when config is not specified, the default is same as parent schema node. If top node does not specify config, default is true.
Added ConfigNode as marker interface for nodes which can contains 'config' statement.
Added AbstractSchemaNodeBuilder as base class for builders of SchemaNode nodes.
Updated tests.

Change-Id: Ibabcf590608bdf482258826fa250692507a71c01
Signed-off-by: Martin Vitez <mvitez@cisco.com>