Add Netbox support

[mweinelt]

Documentation:
https://netbox.readthedocs.io/en/latest/additional-features/webhooks/

Probably need to decide on important fields to identify an object per object type.

Waiting for the the docker container to support webhooks (needs Redis and django-rq) until we can get started capturing events:

• Webhook Backend is not started netbox-community/netbox-docker#88 (comment)
• Working implementation of webhooks using new 'redis' container netbox-community/netbox-docker#90

[fleaz]

Example webhook from Netbox:

Body:
{"event": "updated", "timestamp": "2018-08-21 20:58:04.682320", "model": "device", "data": {"id": 11, "name": "elsa.darmstadt.freifunk.net", "display_name": "elsa.darmstadt.freifunk.net", "device_type": {"id": 7, "url": "/api/dcim/device-types/7/", "manufacturer": {"id": 3, "url": "/api/dcim/manufacturers/3/", "name": "Dell", "slug": "dell"}, "model": "PowerEdge SC1435", "slug": "poweredge-sc1435"}, "device_role": {"id": 3, "url": "/api/dcim/device-roles/3/", "name": "Server", "slug": "server"}, "tenant": {"id": 2, "url": "/api/tenancy/tenants/2/", "name": "NOC", "slug": "noc"}, "platform": null, "serial": "8HR5Z3J", "asset_tag": "44454C4C-4800-1052-8035-B8C04F5A334A", "site": {"id": 1, "url": "/api/dcim/sites/1/", "name": "S2|02 C303", "slug": "s202-c303"}, "rack": {"id": 1, "url": "/api/dcim/racks/1/", "name": "1: FFDA / D120", "display_name": "1: FFDA / D120 (1)"}, "position": 37, "face": {"value": 0, "label": "Front"}, "parent_device": null, "status": {"value": 1, "label": "Active"}, "primary_ip": {"id": 19, "url": "/api/ipam/ip-addresses/19/", "family": 6, "address": "2001:67c:2ed8::4:1/64"}, "primary_ip4": {"id": 18, "url": "/api/ipam/ip-addresses/18/", "family": 4, "address": "82.195.73.4/26"}, "primary_ip6": {"id": 19, "url": "/api/ipam/ip-addresses/19/", "family": 6, "address": "2001:67c:2ed8::4:1/64"}, "cluster": null, "virtual_chassis": null, "vc_position": null, "vc_priority": null, "comments": "VGA Ports defekt, Ger\u00e4t bootet aber noch und serial funktioniert.", "tags": [], "custom_fields": {}, "created": "2017-04-09", "last_updated": "2018-08-21T20:58:04.483195Z"}}
Header:
	Accept-Encoding: [identity]
	Content-Type: [application/json]
	Content-Length: [1609]
	X-Hook-Signature: [c1818cdc320d25eb637758fa98dc4dadf1073aa6a2798b3eac3b5e71f8db13c40175e087fa7e8ed30372254858b6fc83aa591d2a54959b6db309003dfc512b79]
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