fix: switched test config entry that wasnt available for deb9

The --collector.systemd config argument is unavailable in the
older packages.
The formula handles either scenario so switching the check out from the
test pillar for --log.level=debug instead

Resolved issue identified in pull request check for debian

test/integration/repo/controls/service_spec.rb

@@ -44,6 +44,6 @@
 
   describe file('/etc/default/prometheus-node-exporter') do
     its('content') { should include '--web.listen-address=:9110' }
-    its('content') { should include '--collector.systemd' }
+    its('content') { should include '--log.level=debug' }
   end
 end

test/salt/pillar/repo.sls

@@ -81,7 +81,7 @@ prometheus:
           source_hash: b2503fd932f85f4e5baf161268854bf5d22001869b84f00fd2d1f57b51b72424
         environ:
           args:
-            collector.systemd: true
+            log.level: debug
             web.listen-address: ":9110"
         service:
           args: