Allow to mask systemd services

**Is your feature request related to a problem? Please describe.**

I have a service that is enabled by default in Ubuntu and I want to mask it.

**Describe the solution you'd like**

In `config.systemd.services` I'd like to be able to create a service definition and set it to masked.

**Describe alternatives you've considered**

N/A.

**Additional context**

N/A.
