Remove volume_pool_size cinder.conf configuration option

The correct configuration option is pool_size, however this is only used when
using a volume_driver of cinder.volume.drivers.lvm.ThinLVMVolumeDriver, which
we don't support.  We can re-add pool_size if we do decide to support
cinder.volume.drivers.lvm.ThinLVMVolumeDriver.
Merge pull request #75 from mattt416/remove_pool_size_config_option

Merged automatically by jenkins after successful build test