Fix hostname parsing and add tests

[WalBeh]

Summary of changes

dc_util does not work correctly on clusters with multiple sts (eg. like on the previously updated cluster where we added dedicated master nodes).

Checklist

• Link to issue this PR refers to:
• Relevant changes are reflected in CHANGES.rst
• Added or changed code is covered by tests
• Documentation has been updated if necessary
• Changed code does not contain any breaking changes (or this is a major version change)

[tomach]

👍 nice work! maybe in the tests, it might be safer to call extractNodeName() directly instead of re-implementing the parsing logic? then the tests will break if the function ever changes unexpectedly.