RFR: JDK-8195809: [TESTBUG] jps and jcmd -l support for containers is not tested

[Mikhailo Seledtsov]

Please review this addition of new testing:
  - new test for sending jcmd commands across container boundaries
  - updated existing sidecar test, enabling jcmd test case

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Commit messages:
 - Cleanup
 - Adding TestJcmd
 - Made getBaseImageName() and version() public to allow tests access these methods
 - Test JCMD with side car pattern works

Changes: https://git.openjdk.java.net/jdk/pull/5368/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5368&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8195809
  Stats: 201 lines in 3 files changed: 194 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5368.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5368/head:pull/5368

PR: https://git.openjdk.java.net/jdk/pull/5368