make profile_list union of List and Callable

[bitnik]

This PR extends #137 and makes profile_list union of List and Callable.

When profile_list is defined as a callable, it can be customised according to user type or user's auth_state as @clkao already suggested.

Another possibility is that profile list can be retrieved dynamically (probably from a database table) every time user comes to 'Spawner Options' page (no need to helm upgrade every time when profile list has to be updated).

Here is a configuration example for a callable profile_list:

from tornado import gen

@gen.coroutine
def get_profile_list(spawner):
    # define profiles or retrieve from database
    ...
    # update profiles according to user data
    auth_state = yield spawner.user.get_auth_state()
    is_admin = spawner.user.admin
    ...
    return profiles
c.KubeSpawner.profile_list = get_profile_list

It would be nice to hear your opinions!

[minrk]

Looks great, thanks!