Add support for testing different CVMFS setups in the docker images for the pilot.

[terjekv]

For testing CVMFS setups in the pilot, it may be a good idea for the docker images to offer three modes for setting up CVMFS:

1. Bind to stratum 0 (if that fails, we have issues)
2. Bind to a specific stratum 1 to test it, and
3. Go “live” and use Geo-location to pick the closest one.

Selecting one of these modes via an environment variable or similar would allow us to test our CVMFS setup with (relative) ease.

[bedroge]

In principle that should be possible (and I agree that it's a good idea), since you can easily override the CVMFS_SERVER_URL for the client (in this case: the Docker container) by making a .local configuration file. Only the first one may not work, since, afaik, it's quite standard to deny direct access to the Stratum 0 and only allow Stratum 1 servers to connect to it.

[terjekv]

Yeah, I'm not sure we need direct stratum 0 access, but if we did that we'd have to filter a few IP ranges that could be used. But in all honesty, if stratum 0 doesn't work for some reason, I doubt this is how we'd test it. So I supposed only options 2 and 3 are really important.