fix(Makefile): only install data containers if they don't exist

Since we are scheduling data containers to specific machines (and
the machine we choose is random each time), we want to be careful
about when we regenerate and resubmit these units.

This commit makes `make install` (and thus `make run`) safe to
run multiple times again since they will now only submit data containers if they don't
already exist.

TESTING: run `make install` multiple times and ensure the units are there when they should
be, and not resubmitted otherwise
```console
$ make uninstall
```
```console
$ make install
Job deis-router.1.service loaded on e35efc85.../172.17.8.100
Job deis-builder-data.service loaded on e35efc85.../172.17.8.100
Job deis-database-data.service loaded on e35efc85.../172.17.8.100
Job deis-logger-data.service loaded on e35efc85.../172.17.8.100
Job deis-registry-data.service loaded on e35efc85.../172.17.8.100
fleetctl --strict-host-key-checking=false load registry/systemd/deis-registry.service logger/systemd/deis-logger.service cache/systemd/deis-cache.service database/systemd/deis-database.service
Job deis-logger.service loaded on e35efc85.../172.17.8.100
Job deis-registry.service loaded on e35efc85.../172.17.8.100
Job deis-cache.service loaded on e35efc85.../172.17.8.100
Job deis-database.service loaded on e35efc85.../172.17.8.100
fleetctl --strict-host-key-checking=false load controller/systemd/*.service
Job deis-controller.service loaded on e35efc85.../172.17.8.100
fleetctl --strict-host-key-checking=false load builder/systemd/*.service
Job deis-builder.service loaded on e35efc85.../172.17.8.100
```
```console
$ make install
deis-builder-data.service already loaded. Skipping...
deis-database-data.service already loaded. Skipping...
deis-logger-data.service already loaded. Skipping...
deis-registry-data.service already loaded. Skipping...
fleetctl --strict-host-key-checking=false load registry/systemd/deis-registry.service logger/systemd/deis-logger.service cache/systemd/deis-cache.service database/systemd/deis-database.service
fleetctl --strict-host-key-checking=false load controller/systemd/*.service
fleetctl --strict-host-key-checking=false load builder/systemd/*.service
```
Merge pull request #1291 from deis/readable_data_containers

fix(Makefile): only install data containers if they don't exist