Not possible to run two instances of gbuild simulateneously (LXC)

[mjmacleod]

Reproduce:

1. Setup gitian with LXC in two different paths for two different builds.
2. Run each build separately one after the other (works)
3. Run both simultaneously at the same time (doesn't work)

Expected behaviour: Both builds should be able to run simultaneously

Observation: guest port, guest ip and name 'gitian' are all hardcoded, these should probably be configurable by env variable.