RFR 8160063: Provide a means to disable a plugin via the command line

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8160063/webrev.02/ for 
https://bugs.openjdk.java.net/browse/JDK-8160063

PS. Earlier review thread on slightly different fix: 
http://mail.openjdk.java.net/pipermail/jigsaw-dev/2016-September/009392.html

Thanks,
-Sundar