Basic Teamcity agent - only supports guest authentication, default branch

Basic support for the TeamCity CI server (https://www.jetbrains.com/teamcity/) in a similar style to the Jenkins agent check.

Unfortunately I'm not very well-versed in Python testing techniques so I wasn't able to write any tests here. Help with that would certainly be appreciated.
If you use that, `is_deployment` will be a string with "true" or "false". Beware of the following:
```python
>>> t = "true"
>>> f = "false"
>>> bool(t)
True
>>> bool(f)
True
```
So you probably want to cast this flag, you could use `_is_affirmative()` from `config`