RFR(S): 8195809: [TESTBUG] jps and jcmd -l support for Docker containers is not tested

[mikhailo.seledtsov at oracle.com]

Please review this change that:
   - adds test case for "jcmd -l" and "jcmd <pid> help" where jcmd is 
executed on a host/node outside the container,
     while a targeted JVM is running inside a container
   - factors out some common functionality to DockerTestUtils and 
docker.Common

Please note:
   - the "jcmd -l" works in this configuration, however the JCMD's and 
Target's username and UID have to match
     (per design)
   - the "jcmd help", "jcmd JFR.start" or any other JCMD command besides 
"jcmd -l" does not work in this configuration
     (Filed "JDK-8228343: JCMD and attach fail to work across Linux 
Container boundary")
     The test case is commented out, however can be used for reproducing 
the issue, and will be enabled
     once the bug is fixed.


     JBS: https://bugs.openjdk.java.net/browse/JDK-8195809
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8195809.00/
     Testing:
       - ran the new test multiple times on Linux-x64
       - ran TestJCMDWithSideCar multiple times on Linux-x64
       - ran all Docker/Container tests (HotSpot and JDK)
     All PASS

Thank you,
Misha