Correct check for drivers existence in the map. (#940)
Correct check for drivers existence in the map. (#940)

Return default only if requested driver does
not exist in the map. Always returns the driver
 requested.

Signed-off-by: Jose Rivera <4a3487e57d90e2084654b6d23937e75af5c8ee55@portworx.com>
libopenstorage/openstorage,libopenstorage/openstorage,libopenstorage/openstorage