bring snakepit machines online with taskcluster

[bhearsum]

We started doing this in https://mozilla-hub.atlassian.net/browse/RELOPS-575 but hit issues because the host OS is too old. I think our best path forward here is likely to be:

1. Upgrade or reimage to get a more modern Ubuntu version on the host
2. Wait for Consider switching to Docker for all tasks #391 so we run tasks in a docker container

cc @aerickson for visibility or other ideas; it's likely going to be a month or two before we get back around to this though.