Change Marathon provider to make just one API call instead of two per
configuration update by means of specifying embedded resources, which
enable retrieving multiple response types from the API at once. Apart
from the obvious savings in API calls, we primarily gain a consistent
view on both applications and tasks that allows us to drop a lot of
correlation logic. Additionally, it will serve as the basis for the
introduction of readiness checks which require application/task
consistency for correct leverage on the proxy end.
Additional changes:
marathon.go:
- Filter on tasks now embedded inside the applications.
- Reduce/simplify signature on multiple template functions as we do not
need to check for proper application/task correlation anymore.
- Remove getFrontendBackend in favor of just getBackend.
- Move filtering on enabled/exposed applications from `taskFilter` to
`applicationFilter`. (The task filter just reached out to the
applications anyway, so it never made sense to locate it with the
tasks where the filter was called once for every task even though the
result would never change.)
- Remove duplicate constraints filter in tasks, where it neither made
sense to keep as it operates on the application level only.
- Add context to rendering error.
marathon_test.go:
- Simplify and reduce numerous tests.
- Convert tests with high number of cases into parallelized sub-tests.
- Improve readability/structure for several tests.
- Add missing test for enabled/exposed applications.
- Simplify the mocked Marathon server.
marathon.tmpl:
- Update application/task iteration.
- Replace `getFrontendBackend` by `getBackend`.
