Need property to prevent backward compatibility issues for #590 fix

[rodrigo-neira]

Hi @jpbelang , https://github.com/raml-org/raml-java-parser/issues/678 has been solved using a system property to enable strict validation for string values. But https://github.com/raml-org/raml-java-parser/issues/590 has been solved applying strict validation by default, and this generates backward compatibility issues. By default, this boolean validation should be turned off and be able to enable it like for #678 fix.