(maint) Add validation of the facter schema

Previously we've had validation that the facts output by facter
conform to the facter schema, but no catch if the schema itself
was invalid.

This commit adds that additional validation step as a spec test
and to the schema/validate.rb utility script.
Merge pull request #797 from kylog/maint/validate-facter-schema

(maint) Add validation of the facter schema