use os.PathSeparator to support windows path
use os.PathSeparator to support windows path

[#154785759](https://www.pivotaltracker.com/story/show/#154785759)

Signed-off-by: Paul Warren <381ebbe39e97b92332c450533d9eabb9a873db86@emc.com>

cloudfoundry-incubator/volman,cloudfoundry-incubator/volman