RFR(S): 8227122: [TESTBUG] Create Docker sidecar test cases

[mikhailo.seledtsov at oracle.com]

Please review this new test that uses a Docker sidecar pattern to 
manage/monitor JVM running in the main payload container.

Sidecar is a common pattern used in the cloud environments for 
monitoring among other uses. In side car pattern the main 
application/service container that runs the payload is paired with a 
sidecar container. It is achieved by sharing certain namespace aspects 
between the two containers such as PID namespace, specific 
sub-directories, IPC and more.

This test implements the following cases:
   - "jcmd -l" to list java processes running in "main" container from 
the "sidecar" container
   - "jhsdb jinfo" in the sidecar configuration
   - jcmd <some-command>

This change also builds a basis for more test cases in the future.

Minor changes were done to DockerTestUtils:
   - changing access to DOCKER_COMMAND constant to public
   - minor spelling and terminology corrections


     JBS: https://bugs.openjdk.java.net/browse/JDK-8227122
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8227122.00/
     Testing:
         1. ran Docker tests on Linux-x64 - PASS
         2. Running Docker tests in test cluster - in progress


Thank you,
Misha