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

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

Conflicts:
	provider/ec2/ec2.go
	provider/ec2/local_test.go
Merge pull request #1270 from natefinch/backport-def-subnet

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

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

backport of https://github.com/juju/juju/pull/1056

(Review request: http://reviews.vapour.ws/r/584/)