[vSphere] Refactor and extend network interface methods (similar to the ovirt Implementation)

This is the changes discussed with Ohad the other day.
The extend the functionality of vSphere to add/destroy/update the interfaces.
Also a better way to retrieve interfaces was added (interfaces.get).

Changes:
* added methods to add, remove and update interfaces for vSphere
* added missing method to retrieve the interface
* added existing attribute key to interface (required for modifying, deleting interfaces)
* alias interface's mac to id