Fix default parameters value resolution.

[iBavtovich]

Not all custom parameter definitions can be inherited from StringParameterDefinition, but have default String value.
For example ExtendedChoiceParameter:

https://github.com/jenkinsci/extended-choice-parameter-plugin/blob/master/src/main/java/com/cwctravel/hudson/plugins/extended_choice_parameter/ExtendedChoiceParameterDefinition.java

It cannot inherit from StringParameterDefinition, since the method createValue(StaplerRequest req) is final in SimpleParameterDefinition.class

This commit fix default parameters value resolution and makes it dependent on default ParameterValue instead of ParameterDefinition.