Set the System Identifier as a unique string.

Use Case: At installation, it is useful to set a unique identifier (as a system var) that can be used to identify the system. This system var can be then to be used as string mapped to third-party services.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -