RFR(S): 8222769: [TESTBUG] TestJFRNetworkEvents should not rely on hostname command

[mikhailo.seledtsov at oracle.com]

Please review this change that uses a better platform independent way of 
obtaining IP address inside a docker container.
This test worked for Oracle Linux, but did not work on Fedora, which is 
now fixed.
The code for obtaining IP address in getLocalIp() was contributed by 
Severin, thank you Severin.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8222769
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8222769.00/
     Testing:
         Ran the affected test on Linux-x64 (Oracle Linux 7.3, 7.6) - Passed


Thank you,
Misha