Bug 1423 - Yangtools parser allows 2 cases with same child elements

- added check in YangParserImpl if two choice cases childs are same
- rework printSchemaNodeInfo() in GeneratorImpl.xtend, where was
  problem after generating java file with uncompatible types Set<TypeDefinition<? extends Object>>
  instead of Set<TypeDefinition<?>>, problem was fixed adding another
  variable with correct type.

Change-Id: I0a612ea9143c6f29ffe3a8ceef7e703136157641
Signed-off-by: Ladislav Borak <lborak@cisco.com>