The validation as part of #6330 was only for length. This PR adds the
rules for alphanumeric, not having -- within, not ending with a - and
that the id must start with a letter.
The PR also adds tests for these rules
Pull CIDR block validation into a shared func ready to be used elsewhere
Example of new err message:
```
Errors:
* aws_vpc.foo: "cidr_block" must contain a valid network CIDR,
expected "10.0.0.0/16", got "10.0.1.0/16"
```