Allow a min/max volume size to be set on a per-volume-type basis

Many cloud offerings set restrictions on the max/min size of volumes that can be created for different "tiers" of storage - RAX: https://support.rackspace.com/how-to/create-and-attach-a-cloud-block-storage-volume/; Google: https://cloud.google.com/compute/docs/disks/#overview; Amazon: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AmazonEBS.html. In order to support this feature in OpenStack, it makes sense to add the ability to set and enforce size restrictions on a per-volume-type basis, as this is the building block for defining "tiers" of storage. The change will be straightforward - add a specific extra-specs flag for min and max volume size which the API then checks against when creating or retyping volumes. An admin can create these extra-specs flag for any volume type, and if not present no size enforcement will be done. If no default volume type is defined, and no volume type is specified during the create request, no size limit enforcement will be done.