improve error logs due to IllegalArgumentException to include full input

improve error logs due to IllegalArgumentException to include full input

This requires, and adds, a working toString() for all *Request classes,
which is always useful for future logging of requests elsewhere.

This introduction of a NeutronRequest (an ABC instead of an interface)
could in the future also allow a follow-up change to get rid of the
reflection in INeutronRequest, by having a (generic) "singleton" and a
"bulkRequest" field in NeutronRequest instead of in each *Request class.
(The problem with that idea, and why it is how it currently is, are the
@XmlElement annotations for singleton and bulkRequest in each *Request;
those would have to be moved to new getters.)

Change-Id: I9769a3825632dd2bd2f9e2875b91d06a4f781616
Signed-off-by: Michael Vorburger <vorburger@redhat.com>