phosphor-state-manager: create symlinks based on host-instances

As prep work for the multi-host designs, update the target generation
to take into account the OBMC_HOST_INSTANCES value and generate the
targets and links using it.

Tested:

Booted Witherspoon in QEMU and compared /lib/systemd/system before
and after the change.  Obtained the same results in both:

```
root@witherspoon:/lib/systemd/system# find | sort | md5sum
27b2eee499f272cf78b1384ad50a6bb8  -
```

Booted Bletchley in QEMU and confirmed that the undesired @0 targets
are no longer present in the `systemctl list-jobs` after boot.

```
root@bletchley:~# systemctl list-jobs | awk '{ print $2; }' | grep -e service -e target | grep 0
mapper-wait@-xyz-openbmc_project-control-host0-power_restore_policy.service
mapper-wait@-xyz-openbmc_project-control-host0-auto_reboot.service
```

(These remaining ones are induced by xyz.openbmc_project.State.Host,
 which still needs some clean up.)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iff058ecaa74a4cd4d830543b3a9c4fe0305595a8