RFR(S): 8221710: [TESTBUG] more configurable parameters for docker testing

[mikhailo.seledtsov at oracle.com]

These new parameters are introduced to help in development and 
troubleshooting of the Docker tests.


1. Docker command: jdk.test.docker.command
     On some systems docker is installed in locations other than /bin or 
/usr/bin. JTreg harness sets PATH to these locations, hence other 
locations such as /usr/local/bin/ is not visible/executable within JTReg 
tests. A good practice in this case is to provide the full path to the 
executable for the test.

2. Retaining image after test: jdk.test.docker.retain.image
     This is very useful for diagnostic purposes, for trouble shooting. 
By default, docker images created by the tests are removed at the end of 
the test.
     Specifying this option to "true" provides an ability to inspect the 
image, run the image, etc.

3. Overriding JDK under test just for docker tests: 
jdk.test.docker.jdk.under.test
     This feature is useful when developing tests on non-Linux platform. 
In such cases, the default JDK under test is non-Linux, hence will not 
run inside a docker container. This property allows user to point the 
docker tests to JDK-under-test that is built for Linux.

Also, now that jtreg.SkippedException is available started using it.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8221710
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8221710.00/
     Testing: ran docker tests


Thank you,
Misha