Support non-root vdom

[dmitryperets]

By default, it is assumed that the SD-WAN is configured in the "root" VDOM. If this is not the case, it is now possible to specify the VDOM name, using vdom parameter.

This optional parameter can be set either globally (for the entire project):

{% set vdom = "CustX" %}

...or per profile (this overrides the global value):

{% set profiles = {
    'MultiVdomBranch': {
      'options': {
        'vdom': "CustX"
      },
      'interfaces': [
        # ...
      ]
    }
  }
%}

Note that we DO NOT support multiple SD-WAN VDOMs on the same device. For example, we generate several special interfaces with fixed names (such as "Lo", "Lo-HC"...), and this would cause conflicts if you tried to do it in multiple VDOMs.
That is why there must be only one SD-WAN VDOM on the device. It's just that it doesn't have to be the "root" VDOM.

The "root" VDOM is still the default choice, when the vdom parameter is not set (neither in the profile nor globally).