Add priority to interfaces so that file names in interfaces.d can be prioritized

[gbeine]

Debian creates the interfaces following a ranking usually indicated by prepending numbers to the ifcfg-files. This will be enabled by this patch by adding a 'priority' field to interface definitions.

So something like

  bridge:
  - device: lxcbr0
    priority: 20
    type: bridge
...

will result in the filename 20-ifcfg-lxcbr0 instead of ifcfg-lxcbr0.