Implement gnt-cluster check-disk-sizes

This patch adds a new opcode and lu for checking disk sizes. Currently
it does only top-level disk verification, and also doesn't check
primary/secondary node size mismatches (these two are added as TODOs in
the Exec() function of the LU).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>