This provides the following benefits:

1. Build configs under source control.
2. TeamCity admins are no longer the sole owners of build configs.
3. New diff build configs can be modified/created and tested without requiring manual intervention by TeamCity admins. All that is needed is a build bot that listens for a build config name on Phab diffs.
4. Reduced maintenance overhead due to de-duplicating build configs. Our current infra requires manually adding a new build config via TeamCity for every type of build we want to run on diffs. This is separate from the identical builds used for other purposes (ie. master, IBD). Keeping all of these configs up-to-date with eachother is unnecessary.

Each build will be executed by the same TeamCity config of the form:
ABC_BUILD_NAME=<config name> ./build-configurations.sh
ABC_BUILD_NAME will be provided at runtime.
Default build: https://build.bitcoinabc.org/viewLog.html?buildId=16818&buildTypeId=BitcoinABC_Staging_BitcoinAbcDiffTestingMigrationWip&tab=buildParameters
Build without wallet: https://build.bitcoinabc.org/viewLog.html?buildId=16820&buildTypeId=BitcoinABC_Staging_BitcoinAbcDiffTestingMigrationWip&tab=buildParameters
ASAN: https://build.bitcoinabc.org/viewLog.html?buildId=16819&buildTypeId=BitcoinABC_Staging_BitcoinAbcDiffTestingMigrationWip&tab=buildParameters

The script was also run with ABC_BUILD_NAME not set: https://build.bitcoinabc.org/viewLog.html?buildId=16813&buildTypeId=BitcoinABC_Staging_BitcoinAbcDiffTestingMigrationWip&tab=buildLog