Refactor to allow configurable backends (configmap, eks configmap, crd)

[wongma7]

This enables the authenticator server to support multiple backends (sources of IAM->user+group mappings) at the same time with a well-defined order of precedence according to the backend-mode argument.

Fixes #247
also fixes #261

TODO: more tests -- will do in a follow-up
TODO: update README -- done
TODO: fix server_test.go -- done
TODO: sts regional endpoint -- needs some discussion, will do in a follow-up

[k8s-ci-robot]

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[wongma7]

Will put last 2 commits in a separate PR.

Some concerns about whether the user has sts regional endpoints activated or not.

[nckturner]

@wongma7 awesome, yeah I think the regional endpoint is a separate concern.

[micahhausler]

There are a bunch of TODOs, do we want to address those in this PR or at a later time?

Thanks for doing this!

[wongma7]

I think tests can be a follow-up. But I need to update the documentation (and fix the travis build). Will ping when that is ready.

[nckturner]

Looks like we are close 😄

Do we have a test with multiple backends?

[wongma7]

RE: tests there are a couple tests that inadvertently use multiple mappers but no explicit tests yet. https://github.com/kubernetes-sigs/aws-iam-authenticator/pull/269/files#diff-8151a8a5ca3b510f06b426f0cfb48708R646 I would like to refactor server_test.go in a followup as it will be quite verbose to have a configmap+file+crd version of every testcase. Then add some explicit tests.

[nckturner]

/lgtm