Add Cx type power outlets

[raddessi]

Environment

• Python version: 3.6.2
• NetBox version: 2.7.11

Proposed Functionality

There is a proprietary power outlet type introduced by ServerTech that accepts either a C14 or C20 connector, they refer to it as Cx. Article here. It is receptacle only so we would only need to add the outlet, no ports.

Use Case

I would like to be able to doc these outlets without jumping through hoops. I actually wrote up the change and was about to send it as a PR before I saw the instruction to open a FR first. If you wish you can see that branch here

Database Changes

None afaik, the only references to C13 I saw were fields only.

External Dependencies

None

[jeremystretch]

I'm going to add this under a new "proprietary" group. Doesn't seem right to add it under IEC 60320 since it's not part of the spec.

[raddessi]

That's what I had in my change isn't it? https://github.com/netbox-community/netbox/compare/develop...raddessi:master.cx-power-ports?expand=1

[raddessi]

Love it, thanks!