reject changing pools when there are pending decommissions in-progress (#14102)

do not allow mutation to pool command line when there are
unfinished decommissions in place, disallow such scenarios
to avoid user mistakes.

also add testcases to cover all relevant scenarios.

cmd/test-utils_test.go

@@ -2157,6 +2157,7 @@ func mustGetPoolEndpoints(args ...string) EndpointServerPools {
 		SetCount:     setCount,
 		DrivesPerSet: drivesPerSet,
 		Endpoints:    endpoints,
+		CmdLine:      strings.Join(args, " "),
 	}}
 }