[cloudformation] modify elb dynamic with hard-coded AZ list

We have to hard-code these AZs because Fn::GetAZs is causing the
following error:

'The requested Availability Zone us-east-1b is constrainied and cannot
be used together with us-east-1e. Please retry your request by not
choosing us-east-1b and us-east-1e together.'
Merge pull request #15 from signifyd/fix/elb-static-az-list

[cloudformation] modify elb dynamic with hard-coded AZ list