parseyaml in detach-* plugins doesn't work if there is no data

[Matthew Mosesohn]

Detailed bug description:
 By default, there is no data in the additional YAML field, only comments. parseyaml returns undef instead of false as it did before.
Steps to reproduce:
 Install detach-database plugin and deploy 1 controller, 1 database node.
Expected results:
 Deploy successful
Actual result:
 Error: settings_hash_real is not a hash or array when accessing it with remote_keystone at /etc/fuel/plugins/detach-keystone-1.0/hiera-override.pp:27 on node node-4.test.domain.local
Reproducibility:
 Always
Workaround:
 N/A
Impact:
 Affects any parseyaml that tries to evaluate the output
Description of the environment:
 Operation system: <put your information here>
 Versions of components: <put your information here>
 Reference architecture: <put your information here>
 Network model: <put your information here>
 Related projects installed: <put your information here>
Additional information:
 <put your information here>