Stop the route emitter logging directly to stdout

[williammartin]

The route emitter structure has a few fmt.Printlns sprinkled around. This is not ideal for a couple of reasons:

1. When unit testing, expected error messages get printed out to the test stdout, which is confusing when all the tests are actually passing.
2. Having some library depths print to stdio streams is pretty annoying, there's no way to turn it off.

This PR introduces a log to the Emitter struct that can be injected. In tests we then multiwriter to a buffer we can assert on (turns out, these error logs were not actually being tested), and to the GinkgoWriter, so if a test fails we still get the output.

A few further notes on this:

• Errors probably shouldn't be going to stdout at all, but I didn't change it to stderr because that would be a behavioural change.
• It's probably better to have some real smarter logger interface, but that can be changed later.
• Because the eirini codebase favours constructors with private struct fields, unfortunately the log must always be injected, rather than having an opinionated default with a struct field that can be overridden.

This problem is also in the metrics emitter, but I didn't fix it there in case you reject this.

[cfdreddbot]

:white_check_mark: Hey williammartin! The commit authors and yourself have already signed the CLA.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/165100989

The labels on this github issue will be updated when the story is started.

[gdankov]

Thank you for yet another great PR!

We discussed the change and we agree with the points you made. We would also prefer to have an interface for the logger, instead of passing a plain io.Writer. One option (which we already use in some places) is lager.

We'll merge this and throw a chore in our tracker to eventually use lager everywhere.

Feel free to PR the metrics collector code if you wish, preferably with lager.

@gdankov & @JulzDiverse