An identical virtual-cluster name in different sites

[mabazyar]

Issue type

[] Feature request
[X] Bug report
[ ] Documentation

Environment

• Python version: 3.5.2
• NetBox version: v2.3.1

Description

Seemingly, there is no way to have an identical virtual-cluster name in different sites.

How to reproduce the error:

1. Go to virtualization > clusters
2. push "Add" button
3. Create a new cluster under name "test" located in site "testlocation1"
4. Repeat step Flat DCIM API Format #1 and netbox: IP address form button layout #2
5. Create a new cluster under name "test" located in site "testlocation2"

In this use case, the following error comes up:
Cluster with this Name already exists.

[jeremystretch]

There's a strong parallel between this and #238, which addresses a similar restriction on racks/groups.

[jeremystretch]

Folding this into #1943