shared/install: fix DefaultInstance expansion in %i
We should substitute DefaultInstance if the instance is not specified.
Test case:
[Install]
DefaultInstance=bond1
WantedBy= foobar-i-%i.device
$ systemctl --root=/ enable testing4@.service
Created symlink /etc/systemd/system/foobar-i-bond1.device.wants/testing4@bond1.service
→ /etc/systemd/system/testing4@.service.
(before, the symlink would be created as
/etc/systemd/system/foobar-i-.device.wants/testing4@bond1.service)