Fix dependencies issues on `dogwood/3/bare` and `eucalyptus/3/bare`

[madmatah]

Expected Behavior

We should have fully functionnal version of dogwood/3/bare and eucalyptus/3/bare images.

Actual Behavior

Recent builds of dogwood/3/bare and eucalyptus/3/bare flavors are not working anymore.
The installed version of the django-classy-tags dependency is 2.0.0, and this version has dropped support for python 2.x.

In the current state of the project, there is no way to specify custom dependencies for bare flavors to fix this kind of issues.

Proposal

• Add custom requirements.txt to bare flavors and use it in the Dockerfile
• Pin django-classy-tags to 0.8.0 in dogwood/3/bare and eucalyptus/3/bare