Remove unused variable in os-extend api

Temp variable _val in _extend method is not being used, so remove this
'_val' variable.

Change-Id: I4a3f5e28d1882698a3efd116044c4fddff86c474
Closes-Bug: #1267423
Merge "Remove unused variable in os-extend api"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f