Route class: add description field support

[p-semenov-f5]

Could you add description field here?

Original config has it:

net route /Common/external_default_gateway { description "test gw" gw 10.30.0.101 mtu 1501 network default }

Should be like that:

{ "schemaVersion": "1.0.0", "class": "Device", "async": true, "Common": { "class": "Tenant", "external_default_gateway": { "class": "Route", "gw": "10.30.0.101", "network": "default", "mtu": 1501, "remark": "test gw" } } }