ParseUtils.parseAttributeValue should not fail due to DMR type conversions

The task of ParseUtils.parseAttributeValue is to convert a string into a ModelNode. The type of the ModelNode is not particularly important, as later code will do type conversion. And, it is not the job of this method to do validation; there is other code that does that. So, the type conversion this method does should not be allowed to produce failures. Continuing to try the conversion is ok, although not required, but if a conversion can't succeed the pre-conversion ModelType.STRING node should be returned.