support multiple docker parameters and task labels

[tpetr]

Updates Docker parameters and task labels to support multiple values for a specific key. Still testing this out.

Fixes #1074

[ssalinas]

@tpetr it seems the @JsonDeserialize parameter wasn't playing nice with the List<T>. It was trying to deserialize each individual SingularityDockerParameter with the custom class, not the entire list. Serialization was a bit odd also.

In cef1df4 I implemented some 'wrapper' classes for handling the custom (de)serialization. Tested it and the old Map<String, String> as well as the list of parameters was working locally. Let me know what you think of this method of getting it working.

[ssalinas]

Note here, there was not an easy way to do this that wasn't a breaking change (that I could find). So we will need to make it clear in the release notes that while the api will accept Map<String, String> or List<SingularityDockerParameter>, it will always be returning a List<SigularityDockerParameter>, similar for task labels.

[tpetr]

I'm a little hesitant of putting these kinds of exceptions in constructors -- if we somehow had a bogus one saved we'd be throwing every time it was deserialized. What do you think about moving this into SingularityValidator instead?

[ssalinas]

@tpetr moved those checks to the validator. Since I'm setting both objects when the deprecated one is set, validating by making sure that they are the same