[RFE] config.yaml should not be needed for testing

[phracek]

Update meta-test-family, so config.yaml in some cases is not needed.

In case it is already implemented, document it in user_guide section, how developer / tester can use it.

[jscotka]

should be fixed by
#103
in case you not have your config.yaml, or configured via CONFIG envvar, there is used minimal config. it is good for base tests (or modules what does not provide any service)
but you have to set URL=to_some_link to not test default values in minimal config, byut what you want like:

MODULE=docker URL=docker.io/fedora avocado run your_test_or_modulelint

[phracek]

@jscotka @dhodovsk I would like to test just dockerfile linter over Dockerfile. For this case URL is really not needed.

[dhodovsk]

+1 for removing the need for URL from requirements of static linters. User might want to run linters on newly created/not pushed to registry Dockerfiles.