systemd service provider only works on running systems

Puppet Version: 6.14.0
Puppet Server Version:
OS Name/Version: Any systemd-based linux

The systemd service provider "is not functional on this host" in a systemd-nspawn container running without `-boot` or with `-as-pid2`. In these cases there is a separate PID namespace and PID 1 is not systemd. This is because the current "confines" in `providers/service/systemd.rb` checks for "systemd" in `/proc/1/comm`.

Desired Behavior:

Using systemctl to enable/disable/mask units only needs `/usr/bin/systemctl` to exist. If possible I could see having different behavior for handling `ensure` vs `enable` based on PID1. However, when setting up a container image, I should be able to enable/disable/mask services.