switch-to-configuration: Don't restart instances of user@.service
switch-to-configuration: Don't restart instances of user@.service

Restarting user@ instances is bad because it causes all user services
(such as ssh-agent.service) to be restarted. Maybe one day we can have
switch-to-configuration restart user units in a fine-grained way, but
for now we should just ignore user systemd instances.

Backport: 14.04

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