provider/ec2: try next AZ on "no default subnet" error

If the account/region has a default VPC, but one of the
AZs lacks a default subnet, then Juju may get into a
situation where it cannot provision any instances
successfully. We handle the "no default subnet" error
just as we do constrained zones, by trying the next one.

Fixes https://bugs.launchpad.net/juju-core/+bug/1388860