Replace volume size with configured value

Use configured value as volume size unit when
create and extend volume.

NOTE: Some backends don't support the case of "size+1"
(VMAX only works in 8G increments) and the cloud providers
may have their own operation strategies to decide the
allowed input for volume size, such as minimum, maximum,
size unit,  etc.

Change-Id: I2c0e662229aba2281733d092f6b1ca6acea237ee
3 files changed