add disable volume_growth flag (#7196)

weed/command/scaffold/master.toml

@@ -50,6 +50,7 @@ copy_2 = 6                # create 2 x 6 = 12 actual volumes
 copy_3 = 3                # create 3 x 3 = 9 actual volumes
 copy_other = 1            # create n x 1 = n actual volumes
 threshold = 0.9           # create threshold
+disable = false           # disables volume growth if true
 
 # configuration flags for replication
 [master.replication]