BASE_AMI_ variables in static.py

[nfoti]

It would be helpful to make the variables BASE_AMI_{32,64,HVM} into dictionaries instead keyed on zone with the relevant base AMI as a value. I was getting a cryptic error with the resizevolume command and the reason was that an east coast AMI was being used when my volume is on the west coast. I realize I can specify what AMI to use manually, but that seems accident prone when the zone information is known.