Check for multiple documents in YAML files

More than once folk very understandably get confused due to multiple documents in their yaml file.

e.g.

---
a: b
---
c: d

while that is completely legal YAML only the first document is ever loaded.

Make this throw an ERROR in CI:

straylen@aiadm86 ~/a/m/m/ci (doubledocs)> ruby extra-yaml-checks.rb.erb
~/ai-rake/hostgroup-ttaas/data/
ERROR:Multiple (>1) documents found in file "/home/straylen/ai-rake/hostgroup-ttaas/data/hostgroup/ttaas.yaml"
Found YAML errors.

I can't ever think of reason why this is useful and you ever want to do this.