Use named servers

[rcthomas]

The spawner needs to use the named servers option to allow users to have multiple servers spawned at the same time. This can be done by setting an option based on the selected profile.

This line shows how spawn_single_user is called with options from the options form. And this line shows that spawn_single_user is called with a server_name parameter. We can just set this to the profile key.

The config needs to turn on allow_named_servers also.