refactor(retrofit2): replace retrofit client with retrofit2 client

[kirangodishala]

• This PR makes use of retrofit 2.x client in place of retrofit 1.9 and make required changes accordingly.
• Replaced all RetrofitError references with SpinnakerServerException and/or its sub classes.
• All the retrofit1 dependencies and references are removed.
• Retrofit2 boilerplate code was already implemented in kork's kork-retrofit module and this PR makes use of it.

[spinnakerbot]

We prefer that non-test backend code be written in Java or Kotlin, rather than Groovy. The following files have been added and written in Groovy:

• echo-test/src/main/groovy/com/netflix/spinnaker/echo/config/Retrofit2TestConfig.groovy

See our server-side conventions here.

[dbyron-sf]

The above code doesn't have:

.setErrorHandler(SpinnakerRetrofitErrorHandler.getInstance())

so...is there still some RetrofitError-based exception handling? Or maybe there isn't, and we're getting an improvement by moving to ErrorHandlingExecutorCallAdapterFactory....and that improvement is...SpinnakerServerException and children in theory provide better error messages than RetrofitError.

[dbyron-sf]

I'll stop commenting, but...another place with no SpinnakerRetrofitErrorHandler.