Use native network pool for compilation VMs

Rather than pre-allocating a fixed pool of addresses for compilation VMs,
use the address pool as specified in the manifest directly.

The pooling that was done before this change reserved as many networks
as there are concurrent compilation VMs. Upon shutting down one
compilation VM and starting the next, a network could be recycled very
quickly. Quick recycling of networks may lead to problems on EC2, where
termination of a VM does not mean its former network can immediately be
reused.