Add CLA status check if CLA_STATUS_CHECK=true

Without this patch we're not using the CLA app located at
https://cla.puppetlabs.com/api/v1/ to check the status of the CLA
signature for the pull request contributor.

This patch addresses the problem by adding three new configuration
environment variables, `CLA_STATUS_CHECK`, `CLA_API_USERNAME`, and
`CLA_API_PASSWORD`.  If `CLA_STATUS_CHECK` is the string 'true' then the
CLA API will be consulted using the github user id of the pull request
author.