install: allow instantiated units to be enabled via presets

install: allow instantiated units to be enabled via presets

This patch implements https://github.com/systemd/systemd/issues/9421.

The .preset file now is able to take a rule in the format of:(e.g)
enable foo@.service bar0 bar1 bar2

In the above example, when preset-all is called, all three instances of
foo@bar0.service, foo@bar1.service and foo@bar2.service will be enabled.

When preset is called on a single service(e.g: foo@bar1.service), only
the mentioned one(foo@bar1.service) will be enabled.

Tests are added for future regression.