RFR(XS): 8189213: [TESTBUG] Running jtreg tests on machine without docker shows extra message

[mikhailo]

Please review this simple change that makes the error message in 
jtreg-ext/requires/VMProps.java conditional
on java property "vmprops.trace.verbose". W/o this condition an error 
message is printed each time any jtreg
test is ran, including non-docker tests.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8189213
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8189213.00/



======= Background and details:
VMProps.java is called for each test run of jtreg (be it a run 
containing a singe test or multiple tests).
VMProps evaluates the @requires properties. Therefore, in case of this 
bug, any time a user runs any jtreg VM test
on a machine w/o an operational docker engine, this error will pop up.
The fix makes printing of this error message conditional, off by 
default. The message can be turend on
via a property vmprops.trace.verbose when detailed diagnostic info is 
desired.



======== Testing:
Local testing:
   W/o docker present:
     jtreg <arbitrary-jtreg-test>
       No error message
     jtreg -Dvmprops.trace.verbose=true <arbitrary-jtreg-test>
       Detailed error message
     jtreg DockerBasicTest.java
       Test skipped

   With docker installed but no permissions:
     jtreg <arbitrary-jtreg-test>
       No error message
     jtreg -Dvmprops.trace.verbose=true <arbitrary-jtreg-test>
       Detailed error message
     jtreg DockerBasicTest.java
       Test skipped

   With docker installed and operational:
     jtreg <arbitrary-jtreg-test>
       No error message
     jtreg -Dvmprops.trace.verbose=true <arbitrary-jtreg-test>
       No error message
     jtreg DockerBasicTest.java
       Test passed

Testing via automated distributed test system:
   - tier1
   - docker tests
   In progress


Thank you,
Misha