Refine document of how to run end-to-end tests

End-to-end tests are included in the scope of verifying a Flink release from document, and following the read me on github to verify the nightly tests, I found several things need to be clarified, especially on a clean environment (never run nightly tests before), including:

• We should clarify where to find the "<flink dir>" to run the tests against
• We should clarify what preparation required before run the "Kerberized YARN on Docker test", if user choose to.

Besides, I've observed some test failure caused by port occupation like below (probably due to resource release failure of completed tests) but it seems to be environment-related (checked against 4 macbook, 3 with stable failure and 1 success; checked against 2 linux vm, 1 with stable failure and 1 success). Will dig more and add to the document if any environment cleanup/set required, or open another JIRA if any improvement needed for the scripts.

java.net.BindException: Could not start actor system on any port in port range 6123