RFR: 8221340 - [TESTBUG] TestCgroupMetrics.java fails after fix for JDK-8219562

[Bob Vandette]

Please review this change to the Container Metrics implementation.

Change were made in JDK-8219562 [1] to osContainer_linux.cpp to correct
the parsing of of the /proc/self/mountinfo file but corresponding changes to the
Metrics API and Container tests shou,d have been done to match this change. 

BUG: 
https://bugs.openjdk.java.net/browse/JDK-8221340

WEBREV:
http://cr.openjdk.java.net/~bobv/8221340/webrev.00/

TESTING:
These changes were verified by running Mach5  tier1 and 2 tests and the jtreg container 
tests on a Linux x64 system.

[1] https://bugs.openjdk.java.net/browse/JDK-8219562

Bob.