Automate export of DeviceTypes in YAML export

[jeremystretch]

Environment

• Python version: 3.6.8
• NetBox version: 2.7-beta1

Proposed Functionality

Change the built-in DeviceType export format from CSV to YAML, with each DeviceType instance represented as a discrete YAML document

Use Case

Now that YAML-based import is supported, it makes sense to export DeviceTypes as YAML rather than CSV. This should also help people more easily contribute to the community device type library.

Database Changes

None

External Dependencies

None