Explicitly install pulp-server service files

[seandst]

This prevents the pulp_streamer.service from being inappropriately
included in the pulp-server package.

closes #2518
https://pulp.plan.io/issues/2518

[mention-bot]

@seandst, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jortel, @jeremycline and @dkliban to be potential reviewers.

[seandst]

I'm about to do a test build with this spec to make sure the change takes place.

Here's what we have now:

$ rpm -ql pulp-server|grep pulp_streamer.service
/usr/lib/systemd/system/pulp_streamer.service

[seandst]

I came up with a slightly more reassuring test. Here's the pulp-server rpm currently installed in my vagrant env:

$ rpm -ql pulp-server|grep 'pulp_.*.service'
/usr/lib/systemd/system/pulp_celerybeat.service
/usr/lib/systemd/system/pulp_resource_manager.service
/usr/lib/systemd/system/pulp_streamer.service
/usr/lib/systemd/system/pulp_workers.service

And here's the pulp-server rpm that I just built from this PR branch (you can see the commit hash in the rpm name):

$ rpm -qpl pulp-server-2.12.1-0.1.alpha.git.0.7b7883e.fc25.noarch.rpm |grep 'pulp_.*.service'
/usr/lib/systemd/system/pulp_celerybeat.service
/usr/lib/systemd/system/pulp_resource_manager.service
/usr/lib/systemd/system/pulp_workers.service

And then, making sure the pulp_streamer service is still where it ought to be...

On vagrant:

$ rpm -ql python-pulp-streamer|grep pulp_streamer.service
/usr/lib/systemd/system/pulp_streamer.service

In the newly built RPM:

$ rpm -qpl python-pulp-streamer-2.12.1-0.1.alpha.git.0.7b7883e.fc25.noarch.rpm | grep pulp_streamer.service
/usr/lib/systemd/system/pulp_streamer.service

[mhrivnak]

Looks good!