added support for Simple Parameters

Currently the build-flow plugin only supports Boolean and String parameter types. This breaks certain parameters, such as nodelabel, which need the parameter type preserved in order to operate.

This change looks for already defined SimpleParameterDefinitions that have implemented their createValue(String) methods and uses those to create ParameterValues. If the ParameterDefinition isn't a simple one, or doesn't implement createValue(String), then it's handled normally.

Some additional error handling could be added, but I'm successfully using this in production so far. Comments or suggestions are welcome!