[RFE] SSM: Allow optional OS selection for self-scheduling

[sadsfae]

Building on #474 and 189cd16 ensure the following:

• Self-service API request workflows document using http://localhost/api/v3/hosts/os_list and cited as optional
• Self-service API request workflows can optionally request the OS to be provisioned for their cloud and in docs/process as an optional step.

We can include in this RFE general documentation for defining a cloud using --os too as we need to include that still.

Related-to: #487

[grafuls]

This has now been implemented here:
https://review.gerrithub.io/c/redhat-performance/quads/+/1207302

When creating the assignment you can pass ostype like so:

$ curl -k -X POST -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"description": "Short description here", "owner": "tester", "qinq": 1, "vlan": "602", "wipe": "true", "ostype": "RHEL 7.5"}' http://quads2-stage.example.com/api/v3/assignments/self