Addresses #584.

Modifies generation of environment override filenames by providing a dedicated infix constant (rather than deriving the infix portion of the generated docker-compose filenames from the app manifest name, which is now going to deviate anyway). Specifically, now generates docker-compose.env.[environment].yaml instead of docker-compose.kev.[environment].yaml.

Also involves changes to:

• All tests and fixtures to use new naming conventions,
• The Wordpress example files,
• The tutorial files to reflect new filename changes,
• The e2e help Bash script.

Raised as a draft initially as further work is incoming around renaming of kev.yaml to... something else, which will also be handled as part of this PR.