Testsuite: investigate usage of portOffset rather than seding the ports

[rhusar]

Perhaps that would simplify things.

Context: #137 (comment)

[jajik]

There are multiple things:

• Function tomcat_start that allows us to set various ports.
  • Here we could use portOffset feature instead of our manual offsetting.
  • That might interfere with the ability to set ports manually – can we easily apply the offset for some ports only?
    • update: Yes, it possible to add an offset to only some ports (it's done for each Connector in server.xml separately).
• Part of the tomcat docker image script that tries to find an empty shutdown port (here).
  • We want to drop this probably (i.e., drop this section + check testsuite for tomcat_shutdown_port=yes and set it explicitly or use the mechanism above).

[jajik]

I can confirm that portOffset would simplify things. Although, it would be possible to preserve our ability to set ports manually, after going through our test base I would say it is not necessary. I'll prepare a PR once MODCLUSTER-798 gets resolved.