Ignition should be able to enable systemd aliases

[coreosbot]

Issue by @arithx

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Issue Report

Bug

Expected Behavior

When you enable a systemd alias the service should correctly be enabled on the resulting system.

Actual Behavior

Ignition writes a systemd-preset which explicitly does not work with aliases.

Other Information

I ran into this when re-writing the kola nfs tests trying to enable the nfsd.service resulted in nothing happening on the nfs host. Changing the unit being enabled to nfs-server.service fixed the issue.