Improve test coverage of supervisor HTTP API

[brandond]

The request handlers for the supervisor HTTP API (/v1-$PRODUCT/*, /ping, /cacert) along with various helper functions are currently in pkg/server along with other internal controller and startup code. They probably don't belong here, and could be split out for improved maintainability.

While they end up being tested by e2e and integration tests, we don't have any unit test coverage for these functions.

We should:

• Break up pkg/server/router.go into separate domains for router setup, request handlers, and auth helpers
• Move request handlers and router setup into a separate package, move the node password auth helpers in with the rest of the node password stuff
• Add unit tests for all the core routes

There are mocks spread around the nodepassword and etcd/s3 tests that we could probably move into a shared package under tests for reuse.