[TESTBUG] more configurable parameters for docker testing

1. Docker command: jdk.test.docker.command
    On some systems docker is installed in locations other than /bin or /usr/bin. However, 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 built for Linux.

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