Provide a set of roles for normal use cases: Service requestor, request approver, ...

[kastl-ars]

Description of the feature
I am currently struggling with the RBAC concept, due to the many many different permissions available.

Describe the solution you'd like
It would be nice to provide a set of roles that are most likely being used in "normal" use cases.

Example:

• give a user the permissions to "order a service", i.e. create an instance/request
• allow a user to approve an instance/request
• allow a user to create new services
• ...

[Sispheor]

We are completely refactoring the RBAC system for Squest v3. And We'll provide default roles.
I keep this opened until then.