fix: allow set drive count of proper divisible values (#9101)

Currently the code assumed some orthogonal requirements
which led situations where when we have a setup where
we have let's say for example 168 drives, the final
set_drive_count chosen was 14. Indeed 168 drives are
divisible by 12 but this wasn't allowed due to an
unexpected requirement to have 12 to be a perfect modulo
of 14 which is not possible. This assumption was incorrect.

This PR fixes this old assumption properly, also adds
few tests and some negative tests as well. Improvements
are seen in error messages as well.

cmd/endpoint-ellipses_test.go

@@ -104,6 +104,36 @@ func TestGetSetIndexesEnvOverride(t *testing.T) {
 			8,
 			true,
 		},
+		{
+			[]string{"http://host{1...12}/data{1...12}"},
+			[]uint64{144},
+			[][]uint64{{16, 16, 16, 16, 16, 16, 16, 16, 16}},
+			16,
+			true,
+		},
+		{
+			[]string{"http://host{0...5}/data{1...28}"},
+			[]uint64{168},
+			[][]uint64{{12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12}},
+			12,
+			true,
+		},
+		// Incorrect custom set drive count.
+		{
+			[]string{"http://host{0...5}/data{1...28}"},
+			[]uint64{168},
+			nil,
+			10,
+			false,
+		},
+		// Failure not divisible number of disks.
+		{
+			[]string{"http://host{1...11}/data{1...11}"},
+			[]uint64{121},
+			nil,
+			11,
+			false,
+		},
 		{
 			[]string{"data{1...60}"},
 			nil,