systemd module: add option to specify generators

Adding the configuration option 'systemd.generators' to
specify systemd system-generators. The option allows to
either add new system-generators to systemd, or to over-
ride or disable the system-generators provided by systemd.

Internally, the configuration option 'systemd.generators'
maps onto the 'environment.etc' configuration option.
Having a convenience wrapper around 'environment.etc' helps
to group the systemd system-generator configuration more
easily with other 'systemd...' configurations.