Since this resource produces a list it feels more intuitive to give its
attribute a plural name, and since the noun "instance" already means
something specific in the AWS provider that doesn't apply here we use
"names" to indicate that these are availability zone names.
Also includes updating the docs to not show a dynamic count example for
now, since we don't support that yet.
Closing off my other AWS availability zone branch, I'm adding tests for
the existing aws_availability_zones data source.
This closeshashicorp/terraform#4848.
Martin Atkins