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.
timeout 3000 seems huge?! Could we make this shorter so that the checks doesn't hang if there is an issue here.