WIP: Add Consul Service discovery and Dockerize tool

[saswatamcode]

This PR adds an initial implementation of Consul service discovery for Thanos Stores. It makes heavy use of prometheus custom-sd example.

PR also Dockerizes this tool and includes a script to run a test setup using Consul which demonstrates Service Discovery using thanos-sd-sidecar which only selects Thanos Store services from services advertised by Consul. It does this using store.Info() gRPC call.

Steps to run POC setup:

• Checkout add-disc branch after cloning.
• Run make run-consul-setup which will build thanos-sd-sidecar docker image and setup other containers. (Might need to add -u root \ flag in setup.sh from Prometheus docker containers, based on env)
• Check if 9 containers are up (3 Prometheus, 3 Thanos Sidecar, 1 Consul, 1 thanos-sd-sidecar, 1 Thanos Querier)
• Visit Thanos querier at http://localhost:29090/stores and Consul UI at http://localhost:8500/ui
• Curl http://localhost:8000/targets to get targets in Prometheus HTTP SD format
• Run make teardown-consul-setup to teardown entire setup

TODOs:

• Make setup script cleaner.
• Add tests.
• Make Consul implementation similar to current one in prometheus/discovery, unlike custom_sd one.