Add xenorchestra_bonded_network resource

[ddelnano]

This adds the bonded form of the existing xenorchestra_network resource and completes #253. I initially tried to implement this as a single resource that could handle both the bonded and non bonded options, but ran into complications in the implementation. It would have been possible, but this ended up being much cleaner with this approach.

Testing

• New tests pass
• Follow up on VLAN support with bonded networks. The XCP-ng docs claim this is possible, but it doesn't appear to be easily done with the existing APIs (network.createBonded doesn't support a vlan argument). This is accomplished by providing a bonded PIF as an input to the network.create RPC call. This means terraform would model this by having the xenorchestra_network and xenorchestra_bonded_network working in conjunction with each other.
• Add example showing basic usage (simple bonded network) and VLAN bonded network