Add multitenancy support about netbox

[jeremystretch]

Just merged #393, which implements tenancy assignment for:

• Sites
• Racks
• Devices
• VRFs
• VLANs
• Circuits

I have intentionally omitted prefixes and IP addresses, as adding explicit tenant assignment for these objects could be tedious and confusing. For example if I assign a VRF to each of my customers, I don't want to also assign each prefix and IP address within those VRFs to customers. That would be redundant and error-prone.

That said, it would be great if prefixes and IPs could inherit the tenant from their parent VRF (if the VRF has been assigned a tenant). More research is needed to determine whether the overhead required to support this is acceptable.

[jeremystretch]

Extended tenancy support to Prefixes and IPAddresses in e6c06b3. The inheritance seems to work well but needs additional testing.