Update contrib/teamcity documentation with how to build a teamcity agent VM
Summary:
Documented my setting up the TeamCity agents because:
- TeamCity documentation is shit. Some things are implied and many defaults are either not explained, not provided, or both.
- TeamCity errors give no information and look like this: Incompatible Runner: Command Line
- I don't want to have to figure this stuff out again when we migrate away from Debian Stretch.
Test Plan: Diff CI machines are currently running this configuration. Some additional tweaks are required to run IBD and nightly builds, which I will follow up on later.
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2418