Ensure the CI uses a less 'bloated' image

Currently, we're using a buildstream/buildstream-fedora image to install BuildGrid and run our tests on top of.

This is too bloated.

This MR replaced the buildstream/buildstream-fedora image with the python:3.5-stretch image.

• NOTE 1: This image requires that we install setuptools within CI.
• NOTE 2: For now, I have removed Fedora testing as this requires installing Clang which takes a while in CI. Until we maintain our own images, we should avoid installing such large packages in CI.
• NOTE 3: I have also removed some redundant python dependencies declared in setup.py

Related to #39 (closed), but please note that this is just the first step, it does NOT close this issue.

Important Note

Using the python:3.5-stretch image means that we MUST support >= python3.5 in order for CI to pass