clouds: Fix the clouds.yaml locations

[pierreprinetti]

Before this patch, all default locations were scanned for a clouds.yaml even when a match was found. The last found clouds.yaml replaced any previous finding, effectively reversing the order of priority for the locations.

Fixes #3038

[pierreprinetti]

The fix is essentially adding that break statement on line 107.

I am also removing the check on *os.PathError because according to the docs, that's the only error that can ever come from os.Open(). Removing the check lets me write a simpler and more readable conditional.

[coveralls]

coverage: 78.959% (+0.03%) from 78.934%
when pulling 5eb171e on shiftstack:parse_clouds
into f7893b8 on gophercloud:master.

[EmilienM]

LGTM