Group values for variables in a specific environment

[visola]

User Story

As a user I want to group values for variables in a specific environment.

Acceptance Criteria

• [ ] There's an admin endpoint that can be used to create an environment
• [ ] There's an admin endpoint that can be used to select an environment for a specific listener
• [ ] There's an admin endpoint that can be used to set a value for a variable on a specific environment
• [ ] The value for a variable is correctly picked up from the selected environment in the current listener or use the default value if set