Search for version.yaml using path order

Now nailgun searches for version.yaml in this order of priority:
/etc/fuel/version.yaml
/etc/fuel/nailgun/version.yaml
/etc/nailgun/version.yaml

This reflects the modifications made to Fuel library for parsing version file in https://review.openstack.org/85296

Closes-Bug: #1313303

Change-Id: I9005909f0852527d4df422ace80764841ae0fb0e
Merge "Search for version.yaml using path order"