Add support for extend volume in GPFS vol driver

Add support for extend volume support to the GPFS volume
driver.

Ensure that requested volume size is implemented for each
of the volume creation paths.

Replace instances where units.GiB could be used but was not.

Implements: blueprint gpfs-extend-volume-support
Change-Id: I79295205f4147fca109f5cb59497a6e1fbe6d296
Merge "Add support for extend volume in GPFS vol driver"