ifupdownmain: support for marking interfaces as mgmt interfaces

ifupdownmain: support for marking interfaces as mgmt interfaces

All interfaces starting with 'eth' are marked with
ifaceLinkPrivFlags.MGMT_INTF flag

This match can be changed by setting a 'main' module
'module_globals' policy attr 'mgmt_intf_prefix'.
example:
   $cat /var/lib/ifupdown2/policy.d/main.json
   {
    "main": {
        "module_globals" : {
            "mgmt_intf_prefix" : "ensp"
        },
        "defaults": {},
        "iface_defaults": {
            "eth0": {
                "exclude-companion": "mgmt"
            }
        }
    }
  }

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>