BUG-1413: fixed bug in antlr grammar.

Parser was not able to parse unknown node in extension argument declaration.
Added test.

Change-Id: I834cc47bbae8834484ff024f97dcf77f3c5ddbf9
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-1413: fixed bug in antlr grammar."