Added an integration test for fetch module idempotence. (Testing
that validate_checksum is doing what it's supposed to is harder as we'd
have to create a race condition with the downloaded data to trigger it.
Probably need to make that a unittest eventually).
Also give a deprecation message to the validate_md5 parameter so that we
can eventually get rid of it.
This change corrects problems reported by the `yamllint` linter.
Since key duplication problems were removed in 4d48711, this commit
mainly fixes trailing spaces and extra empty lines at beginning/end of
files.
