RFR 8233850: Shenandoah: Shenandoah thread count ergonomics should be container aware

[Zhengyu Gu]

Please review this small patch that uses container-aware 
os::initial_active_processor_count() API for calculating worker thread 
count ergonomics.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233850
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8233850/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu