Allow multiple modules in same garden.yml

[edvald]

This has come up on a few occasions, such as when using multiple Dockerfiles in the same source directory. Shouldn't be hard to support, e.g. by adding a modules key (but still allowing the singular key as a shorthand).

[adnaan]

This would be quite useful. A use case for such a feature would be multiple Dockerfiles for production, development, integration etc.

[eysi09]

This was closed with #535