Boolean values should not be quoted in output

When outputting to YAML or JSON, boolean values should not have quotes around them. An output file will produce E3012 errors when ran through `cfn-lint` because of this. See: https://github.com/awslabs/cfn-python-lint/issues/260 for more details on why its not a warning.

Changing the `validators.boolean` function to return bool values, rather than strings would correct this issue. I can make a PR for this, but I wanted to discuss any possible repercussions first, since all these types would need to be verified.