Using proper volume size for volume creating

Correct the logic that used to find the closet capacity to the requested
volume size. Without this change, for example, end user even couldn't
create an 1GB volume from SL backend by jumpgate.

Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
For cinder volume create, there are different sources for volume create, such as image, snapshot, volume,.... The case
jumpgate volume can handle is only the plain volume create from the backend. In such case, cinder volume API will return 202 upon the REST api, the scheduler and volume driver will find the host, check the quota, and create the volume. If everything goes well, the volume status will be 'active'. Otherwise, it will be marked as 'error'. So most of the
volume create error is reflected via the volume status instead of REST API return code since it is asynchronous call.

For jumpgate volume driver, it is using the synchronous way to simulate the cinder API. The BadRequest is closest 
option, which is similar to the validation code does in the cinder volume api code.