Un-hardcode host/guest IP

I was running into trouble using gitian LXC INSIDE a VirtualBox VM, because
the 10.0.2.x IP range overlaps with the range that VirtualBox uses for
its internal NAT network.

This commit makes it possible to override the host and guest IP that
is used during LXC image creation and the build, using environment variables:

- `GITIAN_HOST_IP` (default=10.0.2.2, host IP from the viewpoint of the
   VM, can be used for both LXC and KVM)
- `LXC_GUEST_IP` (default=10.0.2.5, guest IP from the viewpoint of the
   host, effective for LXC only)

When these are not defined the behavior remains the same as before.
Make it possible to override bridge if for LXC

For example Ubuntu installs a bridge called `virbr0` by default for LXC.
Using this system-provided bridge avoids having to configure one
specifically before gitian building.

This commits makes it possible to override the bridge used by gitian
using the environment variable LXC_BRIDGE. The default is still to use
`br0` as before.
Merge pull request #52 from laanwj/master

Un-hardcode host/guest IP