Local setup for end2end tests

[pcapriotti]

This introduces a replacement for integration.sh, written in python, that is able to run two (or more) backends at the same time. It works by creating configuration files on the fly for a set of services, modifying them so that they work together on a new set of ports.

TODO

• Set up local SRV records
• Add spawn timeout
• Add a second cannon instance
• Spawn only one backend when INTEGRATION_FEDERATION_TESTS is not set
• db-reset should reset the second test keyspace as well
• Remove legacy integration.sh script
• The two backends should use different elasticsearch indices

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[stefanwire]

Just a nit left.