Previously the 'flake8' command was running but didn't seem to
check out something in all cases.
- This patch decouples the 'ansible-lint' and the 'flake8' sections
into separated 'testenv'.
- And this patch fixes all the 'flake8' issues.
Change-Id: I30169598d086515121175ca322882ee00f05b913
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>