RFR: JDK-8146002: Need to support mirrors for bootstrapping Jib

[Erik Joelsson]

Hello,

If we are to rely on Jib, the whole process needs redundancy in download 
locations. For the main downloads, this is already in place, but the 
weak link is the initial bootstrapping of the tool itself. I propose to 
add support in common/bin/jib.sh for checking a list of mirrors to 
download the install script from.

Bug: https://bugs.openjdk.java.net/browse/JDK-8146002
Webrev: http://cr.openjdk.java.net/~erikj/8146002/webrev.01/

/Erik