Fix Coordinator to not use period in hostname as namespace

[BenediktBurger]

Coordinator did turn the full hostname (including periods, for example "hostname.domain.tld") into the namespace.
A namespace must not contain periods.
This PR adds tests (which failed even where gethostname returns just the last name) and a fix covered by these tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.60%. Comparing base (646ccfd) to head (6867ff6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   87.60%   87.60%           
=======================================
  Files          36       36           
  Lines        2888     2888           
  Branches      348      348           
=======================================
  Hits         2530     2530           
  Misses        298      298           
  Partials       60       60           

Flag	Coverage Δ
unittests	87.60% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.