Add support for specific network configuration

The networkconfig plugin can now handle custom NetworkDetails
objects that contain data like IP address, gateway, broadcast etc.

Change-Id: I16b0b5617737af25970176ca44d8c8146e8c7f96
Add support for OpenNebula meta-data

All the needed details are taken from a context script file
found on a ISO mounted inside the VM.

Change-Id: Idd7e38636a3eaf98a9eb2e4429b371317f29d7e4