make allowed_roles key more descriptive

[sverhoeven]

In PR #56 roles where introduced. Given an application has a certain role then a user with that role is allowed to submit jobs to it.
The role names are now configured with allowed_roles key in the application section of the config file.
The keyallowed_roles can be misinterpreted.

From #56 (review)

In the configuration file, "allowed_roles" sounds like "given the application, you are allowed to have this role", 
whereas strictly speaking it is "given a role, you are allowed to use the application." 
Perhaps "authorized_roles" is better? Or change to "groups" and use "authorized_groups"?

We should find a more descriptive name for allowed_keys. For example:

1. authorized_roles
2. required_roles
3. roles
4. user_allowed_roles
5. granted_roles
6. ...