Use CircleCI for some builds

[exarkun]

CircleCI is a CI-as-a-Service similar to TravisCI. The Tahoe-LAFS project already uses TravisCI for some CI tasks. However, Tahoe-LAFS uses (and maintains) a BuildBot? deployment for some other CI tasks.

BuildBot? is used to run the test suite on the following platforms:

• CentOS 7
• Debian Jessie
• Debian Stretch
• Fedora 24
• OpenBSD 5.9/amd64
• OS X 10.13
• OpenBSD 6.3/amd64
• Ubuntu Xenial (16.04)
• raspbian "jesse"/arm

There are also BuildBot? jobs for checking against pending upstream deprecations and building tarballs.

TravisCI offers a couple or a few specific Linux distros for the build environment - mostly without overlap with the above list. It does offer support for Docker and so it would be possible to bootstrap a test environment for some of the Linux distributions in the above list. This would not help with the non-Linux platforms which cannot be run in a Docker container.

However, CircleCI offers first-class support for custom Docker images which makes it easier to test with alternate Linux distros. Unfortunately the cloud version doesn't offer support for custom VM images so non-Linux platforms are still out of luck.

CircleCI also offers macOS support (with a usage limit).

Compared to CircleCI, BuildBot? is more effort to maintain. Slaves present an _uninterestingly_ complex diversity of configurations (e.g., having tox 2.3.9 instead of 2.7.0 really does not matter to Tahoe-LAFS as far as any user-facing functionality is concerned: it only makes it harder to manage the CI system). Slaves are harder to update because each is a unique snowflake.

Therefore, it would be beneficial to move the Linux-based BuildBot? builders on to CircleCI.

All of the Linux platform test-running builders can likely be moved to CircleCI and retired.

[exarkun]

CircleCI configuration added in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/814203961d935b4b010fe62575445d9f332b8514

Redundant BuildBot? builders reconfigured to only run tests not running on CircleCI now.