BUG-1305: improved antlr grammar handling of unknown nodes.

Removed useless unknown_statement2 and unknown_statement3 definitions

Improved antlr parser to propertly handle extension description by moving unknown_statement definition as last
statement in extension_stmt definition to properly handle description.

Change-Id: I9d8da69ac9336feee07855e85fad5ee2e4033f12
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-1305: improved antlr grammar handling of unknown nodes."