Implement disk grow at LU level

This patch adds a new opcode and LU for growing an instance's disk.

The opcode allows growing only one disk at time, and will throw an error
if the operation fails midway (e.g. on the primary node after it has
been increased on the secondary node). As such, it might actually leave
different sized LVs on different nodes, but this will not create
problems.

Reviewed-by: imsnah