Add a UUID string validator

The `uuid` validator uses a regex validator to match on all UUID
strings allowed by the `uuid.UUID` class.
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Signed-off-by: Jimmy Thrasibule <thrasibule.jimmy@gmail.com>
Fix UUID validator implementation

- Document the new `flags` parameter now accepted by the Regex
  validator.
- Make use of the correct regex flag.
Add unit tests for UUID validator
Add the `uuid` validator to the API documentation
Fix test unit errors

Replace assertIsNone by assertEqual.
Merge pull request #201 from spack/feature.uuid-validator

Feature:  UUID validator