capacity field in NodeExpandVolumeResponse is optional even for node only expansion

[gnufied]

Since capacity field in NodeExpandVolumeResponse is optional, it presents a problem for CSI driver that only implement node side expansion. In that case, if a CSI driver does not report size after expansion, it is kind of hard to know exact size of the volume after expansion.

If expansion succeeded we do know volume is at least of requested capacity size, but we may not know the actual capacity after resizing.