[python] feature: upgrade packages when deploying

[lucasts]

enable pip to upgrade packages which changed since last deploy. also, will avoid cleaning up app's images manually

• Warning *

This can lead to problems to apps careless about their packages versions.

[andrestc]

LGTM but would like to have more opnions regarding the point mentioned in the warning. In my opnion we should favor users that follow the best practices (those would not have problems, if I understood correctly).

[lucasts]

@andrestc agreed. Tsuru servers admins can choose to warn users about this change and allow them some time to check and update their requirements/setup if necessary.

[ggarnier]

I agree. Correct if I'm wrong, but if someone run into problems with this, he can fix it by setting specific dependency versions in requirements.txt.

[lucasts]

@ggarnier that's correct.