patch in preparating for Eucalyptus driver support

[sang-min . park]

This patch contains changes that was necessary for subclassing Euca driver from 
EC2.
The most notable changes are to use class variables instead of class-instance 
variables in various places such that inheritance works correctly.
In EC2 driver, the differences b/w EC2 and Euca are factored into additional 
methods.
The actual Eucalyptus driver will be sent shortly.