Align pingable: documentation, validation & unit testing with RFC 5943

As reported by @toreanderson on db-wg, the documentation describing valid values for the pingable: attribute made no sense and was not alligned with RFC5943. 

I've removed the reference to RoutePrefixSyntax() because the attribute should not be parsed as a prefix but as either a single IPv4 or IPv6 address. 

Groovy tests have been updated accordingly. 

The build is failing on travis, but I don't think this is my fault, I have not touched any API code.