spec: Volume from snapshot with different size

There's no mention in the spec to expected behavior when the
required_bytes in CreateVolumeRequest's capacity_range is larger or
smaller than that of the VolumeContentSource.

This may lead to different interpretations on the behavior.

We are now explicit about expected behavior. If it's smaller we fail
with OUT_OF_RANGE, and if it's greater the volume must be extended by
the driver if necessary.

Author: Akrog

csi.proto

@@ -182,7 +182,11 @@ message CreateVolumeRequest {
 
   // This field is OPTIONAL. This allows the CO to specify the capacity
   // requirement of the volume to be provisioned. If not specified, the
-  // Plugin MAY choose an implementation-defined capacity range.
+  // Plugin MAY choose an implementation-defined capacity range. If
+  // specified it MUST always be honored, even when creating volumes
+  // from a source; which may force some backends to internally extend
+  // the volume after creating it.
+
   CapacityRange capacity_range = 2;
 
   // The capabilities that the provisioned volume MUST have: the Plugin