Fixes #149 (Cleanup umount in ProjectHanlon::ImageService::Base)

The changes in this PR clean up 'umount' method so that the appropriate method is used to unmount
the tmp volume created when adding images (`fusermount` if `fuseiso` was used to mount and `umount` if `mount` was used to mount). There shouldn't be any changes in functionality due to this PR (from the end-user's point of view), but it will prevent failed attempts to `umount` a volume that was mounted using the `fuseiso` command (or attempts to `fusermount -u` a volume that was mounted using the `mount` command).