Better service account outputs in project module

[ludoo]

This change rationalizes the way robot and default service accounts are exposed in the project module's outputs:

• all service accounts are exposed via a single service_accounts output
• the general/legacy service robot is exposed in service_accounts.cloud_services
• per-service robots are exposed in service_accounts.robots, eg service_accounts.robots.container-engine
• default service accounts are exposed in service_accounts.default, eg service_accounts.default.compute

Adding extra robot service accounts is done via the local.service_accounts_robot_services variable in the service_accounts.tf file which has been added in this PR.

[ludoo]

@drebes given that you're OOO for a while I'm merging this. :) Feel free to bug me about it of course when you come back.