provider/ec2: match more constrained AZ errors

Horacio reports a bootstrap failure caused by
a constrained AZ, but received a different error
message from EC2:

"2014-06-17 13:35:22 ERROR juju.provider.common bootstrap.go:119 bootstrap failed:
cannot start bootstrap instance: cannot run instances: Your requested instance type
(m3.medium) is not supported in your requested Availability Zone (us-east-1a).
Please retry your request by not specifying an Availability Zone or choosing
us-east-1e, us-east-1c, us-east-1d. (Unsupported)"

The change I've made is simply to match any "Unsupported" error
that contains the string "Availability Zone".

Fixes https://bugs.launchpad.net/juju-core/+bug/1330163