Make buildbot an optional dependency under all

Review Request #10156 — Created Sept. 21, 2018 and submitted — Latest diff uploaded

Information

ReviewBot
master
eec4285...

Reviewers

The new "all" optional dependency target is intended to replicate the
old behavior of installing all python dependencies.

Documentation has been updated to reflect this.

Since this is just a change for pip dependencies I've done:
- pip install -e ./bot
- pip install -e ./bot[all]

To test if it installs and doesn't install buildbot under the default
package and the "all" target, and according to the logs this is the case.