release_tool: Include changelog when deciding what to release.
release_tool: Include changelog when deciding what to release.

It's easier to read than the git history, which often contains a large
amount of commits to testing.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

pasinskim/integration,pasinskim/integration,pasinskim/integration