Framework should not validate region/zone combinations

This makes it difficult to add new regions to the automated testing setup.

+ execute-ubuntu-ec2-test -r us-west-2 -s ebs -b 20111110 -a i386 -R oneiric -t tests simple-user-data
INFO:root:Creating test for size:m1.small for arch:i386 in region:us-west-2 placement:any root storage:ebs
INFO:root:Retieving ec2 AMI informfation from http://uec-images.ubuntu.com/query/oneiric/server/daily.txt
INFO:root:Found AMI ami-16f77a26
Traceback (most recent call last):
  File "/usr/bin/execute-ubuntu-ec2-test", line 148, in <module>
    creds[settings.SECRET_ACCESS_KEY])
  File "/usr/share/ubuntu-server-ec2-testing/python/ubuntu/ec2/testing.py", line 240, in __init__
    settings.REGION_PLACEMENT_LOOKUP[self.image.region]):
KeyError: 'us-west-2'
Archiving artifacts

ec2 will error is someone does something crazy so suggest these checks are removed