Update coveralls config

[thrau]

fixes #4200

[thrau]

slow build times in docker could have the same reason as this issue: pytest-dev/pytest-cov#159

[thrau]

still one issue left to get it over the line:
the coverage database includes the absolute file paths to code files. the path inside the docker containers are /opt/code/localstack/..., once copied outside of the container, the files are under a different path and therefore the coverage data isn't reported correctly.

symlinking doesn't work, coverage doesn't appear to follow symlinks.

[thrau]

Coverage results are not correct in coveralls now... Maybe relative paths will work: nedbat/coveragepy#597

[thrau]

🎉 coverage results are back online! https://coveralls.io/builds/40929854

[whummer]

Awesome 🎉 , thanks @thrau !!