Add integration tests which test happy path

This PR adds simple go based integration tests, which can run Loki
in two modes:

- single-binary
- micro-services with index-gateway

After standing up it performs the most basic duties like:
- ingest logs
- run a range query
- query label names and label values

This is meant to catch more significant problem as early as possible in
the PR.

We should aim to keep this as simple as possible.