Define JupyterHub admin users in user config yaml file

[Pierre Poulain]

For instance:

plasma_groups:
  - group_1
  - group_2
  - group_3

users:
  - name: foo
    password: PLAIN_TEXT_PASSWORD
    groups:
      - group_1
      - group_2
    admin: true

  - name: bar
    password: PLAIN_TEXT_PASSWORD
    groups:
      - group_3

[Jeremy Tuloup]

The motivation for this issue is to keep most of the user configuration in one place.