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>
Just curious -- does this 400 response match upstream behavior if I were to give normal Cinder some absurd value for volume size?