Document versions involved in instanceId backward compatibility code.
Document versions involved in instanceId backward compatibility code.

Also, make instanceId private (since it can no longer be final), as allowing
it to be modified from outside of EC2Slave itself would cause serious problems.

ikikko/ec2-plugin,databricks/ec2-plugin,Vlatombe/ec2-plugin,jenkinsci/ec2-plugin,mtolan/ec2-plugin,mtolan/ec2-plugin,Khan/ec2-plugin,ydubreuil/ec2-plugin,barretts/ec2-plugin,fengxx/ec2-plugin,mtolan/ec2-plugin,arcivanov/ec2-plugin,Vlatombe/ec2-plugin,ikikko/ec2-plugin,vassilevsky/ec2-plugin,kunickiaj/ec2-plugin,Vlatombe/ec2-plugin,jenkinsci/ec2-plugin,Khan/ec2-plugin,mkozell/ec2-plugin,vassilevsky/ec2-plugin,jenkinsci/ec2-plugin,mkozell/ec2-plugin,vassilevsky/ec2-plugin,kunickiaj/ec2-plugin,arcivanov/ec2-plugin,cyrille-leclerc/ec2-plugin,fengxx/ec2-plugin,fengxx/ec2-plugin,mkozell/ec2-plugin,ikikko/ec2-plugin,Khan/ec2-plugin,cyrille-leclerc/ec2-plugin,hudson3-plugins/ec2-plugin,fengxx/ec2-plugin,Vlatombe/ec2-plugin,hudson3-plugins/ec2-plugin,mkozell/ec2-plugin,hudson3-plugins/ec2-plugin,hudson3-plugins/ec2-plugin,kunickiaj/ec2-plugin,ydubreuil/ec2-plugin,kunickiaj/ec2-plugin,ydubreuil/ec2-plugin,databricks/ec2-plugin,barretts/ec2-plugin,barretts/ec2-plugin,vassilevsky/ec2-plugin,databricks/ec2-plugin,Khan/ec2-plugin,ydubreuil/ec2-plugin,arcivanov/ec2-plugin,databricks/ec2-plugin,cyrille-leclerc/ec2-plugin,ikikko/ec2-plugin,jenkinsci/ec2-plugin,mtolan/ec2-plugin,arcivanov/ec2-plugin,cyrille-leclerc/ec2-plugin,barretts/ec2-plugin