Allow the addition of a "bastion" network to the admin node

If the network proposal contains a configuration of a bastion network as
e.g. documented in
https://github.com/crowbar/crowbar/wiki/Editing-the-network-json
automatically allocate an IP for the admin node in that network during
installation. This is helpful if you need to run the installation via ssh but
cannot use the admin network for that.
Merge pull request #135 from rhafer/fix/bastion-network

Allow the addition of a "bastion" network to the admin node