[BUG] Validate owner in self-assignment API

[athiruma]

• QUADS version (rpm -qa | grep quads and/or quads --version):
• Python version:
• Operating System:

Describe the bug A clear and concise description of what the bug is. curl -k -X POST -H "Authorization: Bearer $TOKEN" -H "Content-Type: application/json" -d '{"description": "Short description here", "owner": "hello_quads", "qinq": 1, "vlan": "602"}' http://quads2-stage.example.com/api/v3/assignments/self

The owner must be a valid user, nice to have user validation.

To Reproduce Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Generate TOKEN for test In payload: input owner with any random word.

Expected behavior A clear and concise description of what you expected to happen. The owner must be an active or valid user.

Screenshots If applicable, add screenshots to help explain your problem.

Additional context Add any other context about the problem here.