Docker auth config

[jaredstehler]

I figured the easiest way to get this done was simply to use the AuthConfig class from the spotify docker client, which has @JsonProperty annotations already; downside is they are capitalized..

  @JsonProperty("Username") private String username;
  @JsonProperty("Password") private String password;
  @JsonProperty("Email") private String email;
  @JsonProperty("ServerAddress") private String serverAddress;

also not a huge fan of the test I wrote b/c of the reflection.

Fixes #1098

[ssalinas]

Hey @jaredstehler , sry missed this when it was first opened. Can you fix the merge conflicts and I can get this merged through our environments and into master?

[tpetr]

I thought I had commented already but I must have forgotten to hit Submit or something -- I'd prefer it if we used a custom POJO for these Docker auth fields (something like SingularityDockerAuthConfiguration) so that 1) we stick with a consistent naming scheme and 2) we're shielded from breaking changes from this Docker client library.

[jaredstehler]

@ssalinas resolved merge conflicts
@tpetr updated to use custom pojo for docker auth config