Support variable server pools (#11256)

Current implementation requires server pools to have
same erasure stripe sizes, to facilitate same SLA
and expectations.

This PR allows server pools to be variadic, i.e they
do not have to be same erasure stripe sizes - instead
they should have SLA for parity ratio.

If the parity ratio cannot be guaranteed by the new
server pool, the deployment is rejected i.e server
pool expansion is not allowed.

cmd/xl-storage_test.go

@@ -1152,7 +1152,7 @@ func TestXLStorageReadFile(t *testing.T) {
 	for l := 0; l < 2; l++ {
 		// 1st loop tests with dma=write, 2nd loop tests with dma=read-write.
 		if l == 1 {
-			globalStorageClass.DMA.DMA = storageclass.DMAReadWrite
+			globalStorageClass.DMA = storageclass.DMAReadWrite
 		}
 		// Following block validates all ReadFile test cases.
 		for i, testCase := range testCases {
@@ -1212,7 +1212,7 @@ func TestXLStorageReadFile(t *testing.T) {
 	}
 
 	// Reset the flag.
-	globalStorageClass.DMA.DMA = storageclass.DMAWrite
+	globalStorageClass.DMA = storageclass.DMAWrite
 
 	// TestXLStorage for permission denied.
 	if runtime.GOOS != globalWindowsOSName {