provisioner: augment Networks config from caller when -unsafe (#411)

Currently container instances are hard-coded to join a single network,
the network associated with the session.

This change allows the caller of CreateInstance to specify which
additional networks should be joined. This is useful, for example, when
a container instance requires access to additional "backend" services
that may be running.

There are security implications associated with this change, hence the
additional networks are only joined when a new -unsafe flag is
specified. It is hoped the name is a sufficient indicator that thought
needs to go into using it.

pwd/types/instance.go

@@ -40,4 +40,5 @@ type InstanceConfig struct {
 	Type           string
 	DindVolumeSize string
 	Envs           []string
+	Networks       []string
 }