Fix bug 2821 - restconf RPC parsing returns http status 500

Fix bug 2821 - restconf RPC parsing returns http status 500

Note: RpcDefinition is a direct child of SchemaNode and we are not able to
use it as DataSchemaNode or SchemaContext. In next RpcDefinition hasn't any
child except Input & Output ContainerNodes. So we are using RpcDefinition
for SchemaNode as a marker in InstanceIdentfierContext.

* json input read @Provider has to change RpcDefinition SchemaNode
to Input ContainerSchemaNode for a correct parsing RPC input data from stream

* no-payload Rpc - Rpc Call could be without payload, but jersey always does
to parse empty stream to NormalizedNode so we add check for stream.available
before start parsing input NormalizedNode from stream and return NNContext
with null data. All checks for data exist have to be implemented for every
method with NormalizedNodeContext.

* fix RpcOutput NormalzidNodeContext in RestconfImpl - we expect to have
RpcDefinition as reultNodeSchema and data are payloaded inside as Output obj.

* fix NormalizedNodeJsonBodyWriter and NormalizedNodeXmlBodyWriter for RpcDefinition
- RpcDefinition has no child elements so RpcDefinition.output has to be a root
DataSchemaNode for json parser

* add JsonBodyReader/Writer test suite
* add Xml rpc Writer parser test suite

Change-Id: I31273bee937f57fa1551a33fcc4d6c81b984afb6
Signed-off-by: Vaclav Demcak <vdemcak@cisco.com>