[haproxy] Declare haproxy in the services tuple

[suraj-cmd]

setup() calls add_service_status() and add_journal() for the haproxy unit
rather than declaring it in the services tuple.
Plugin._collect_services() already runs each entry of that tuple through
is_service() and calls both, so the explicit calls are redundant.

Declaring the unit also gives the plugin an additional enablement trigger.
haproxy currently enables on the package or on the puppet-generated config
directory, so a host running the service without the package installed locally
is missed.

haproxy is the only plugin in the tree still using the explicit form for a unit
it collects both status and journal for.

This follows @TurboTurtle's review comment on #4429.

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo dnf install -y 'dnf*-command(copr)'
• dnf copr enable packit/sosreport-sos-4436
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.