extend server config.yaml to support per pool set drive count (#19663)

This is to support deployments migrating from a multi-pooled
wider stripe to lower stripe. MINIO_STORAGE_CLASS_STANDARD
is still expected to be same for all pools. So you can satisfy
adding custom drive count based pools by adjusting the storage
class value.

```
version: v2
address: ':9000'
rootUser: 'minioadmin'
rootPassword: 'minioadmin'
console-address: ':9001'
pools: # Specify the nodes and drives with pools
  -
    args:
        - 'node{11...14}.example.net/data{1...4}'
  -
    args:
        - 'node{15...18}.example.net/data{1...4}'
  -
    args:
        - 'node{19...22}.example.net/data{1...4}'
  -
    args:
        - 'node{23...34}.example.net/data{1...10}'
    set-drive-count: 6
```

cmd/server-main.go

@@ -18,6 +18,7 @@
 package cmd
 
 import (
+	"bytes"
 	"context"
 	"encoding/hex"
 	"errors"
@@ -287,23 +288,7 @@ func serverCmdArgs(ctx *cli.Context) []string {
 	return strings.Fields(v)
 }
 
-func mergeServerCtxtFromConfigFile(configFile string, ctxt *serverCtxt) error {
-	rd, err := Open(configFile)
-	if err != nil {
-		return err
-	}
-	defer rd.Close()
-
-	cf := &config.ServerConfig{}
-	dec := yaml.NewDecoder(rd)
-	dec.SetStrict(true)
-	if err = dec.Decode(cf); err != nil {
-		return err
-	}
-	if cf.Version != "v1" {
-		return fmt.Errorf("unexpected version: %s", cf.Version)
-	}
-
+func configCommonToSrvCtx(cf config.ServerConfigCommon, ctxt *serverCtxt) {
 	ctxt.RootUser = cf.RootUser
 	ctxt.RootPwd = cf.RootPwd
 
@@ -331,8 +316,75 @@ func mergeServerCtxtFromConfigFile(configFile string, ctxt *serverCtxt) error {
 	if cf.Options.SFTP.SSHPrivateKey != "" {
 		ctxt.SFTP = append(ctxt.SFTP, fmt.Sprintf("ssh-private-key=%s", cf.Options.SFTP.SSHPrivateKey))
 	}
+}
 
-	ctxt.Layout, err = buildDisksLayoutFromConfFile(cf.Pools)
+func mergeServerCtxtFromConfigFile(configFile string, ctxt *serverCtxt) error {
+	rd, err := xioutil.ReadFile(configFile)
+	if err != nil {
+		return err
+	}
+
+	cfReader := bytes.NewReader(rd)
+
+	cv := config.ServerConfigVersion{}
+	if err = yaml.Unmarshal(rd, &cv); err != nil {
+		return err
+	}
+
+	switch cv.Version {
+	case "v1", "v2":
+	default:
+		return fmt.Errorf("unexpected version: %s", cv.Version)
+	}
+
+	cfCommon := config.ServerConfigCommon{}
+	if err = yaml.Unmarshal(rd, &cfCommon); err != nil {
+		return err
+	}
+
+	configCommonToSrvCtx(cfCommon, ctxt)
+
+	v, err := env.GetInt(EnvErasureSetDriveCount, 0)
+	if err != nil {
+		return err
+	}
+	setDriveCount := uint64(v)
+
+	var pools []poolArgs
+	switch cv.Version {
+	case "v1":
+		cfV1 := config.ServerConfigV1{}
+		if err = yaml.Unmarshal(rd, &cfV1); err != nil {
+			return err
+		}
+
+		pools = make([]poolArgs, 0, len(cfV1.Pools))
+		for _, list := range cfV1.Pools {
+			pools = append(pools, poolArgs{
+				args:          list,
+				setDriveCount: setDriveCount,
+			})
+		}
+	case "v2":
+		cf := config.ServerConfig{}
+		cfReader.Seek(0, io.SeekStart)
+		if err = yaml.Unmarshal(rd, &cf); err != nil {
+			return err
+		}
+
+		pools = make([]poolArgs, 0, len(cf.Pools))
+		for _, list := range cf.Pools {
+			driveCount := list.SetDriveCount
+			if setDriveCount > 0 {
+				driveCount = setDriveCount
+			}
+			pools = append(pools, poolArgs{
+				args:          list.Args,
+				setDriveCount: driveCount,
+			})
+		}
+	}
+	ctxt.Layout, err = buildDisksLayoutFromConfFile(pools)
 	return err
 }