Add new site config sub-system intended to replace region (#13672)

- New sub-system has "region" and "name" fields.

- `region` subsystem is marked as deprecated, however still works, unless the
new region parameter under `site` is set - in this case, the region subsystem is
ignored. `region` subsystem is hidden from top-level help (i.e. from `mc admin
config set myminio`), but appears when specifically requested (i.e. with `mc
admin config set myminio region`).

- MINIO_REGION, MINIO_REGION_NAME are supported as legacy environment variables for server region.

- Adds MINIO_SITE_REGION as the current environment variable to configure the
server region and MINIO_SITE_NAME for the site name.

cmd/auth-handler.go

@@ -299,7 +299,7 @@ func checkRequestAuthTypeCredential(ctx context.Context, r *http.Request, action
 		}
 		cred, owner, s3Err = getReqAccessKeyV2(r)
 	case authTypeSigned, authTypePresigned:
-		region := globalServerRegion
+		region := globalSite.Region
 		switch action {
 		case policy.GetBucketLocationAction, policy.ListAllMyBucketsAction:
 			region = ""
@@ -529,7 +529,7 @@ func validateSignature(atype authType, r *http.Request) (auth.Credentials, bool,
 		}
 		cred, owner, s3Err = getReqAccessKeyV2(r)
 	case authTypePresigned, authTypeSigned:
-		region := globalServerRegion
+		region := globalSite.Region
 		if s3Err = isReqAuthenticated(GlobalContext, r, region, serviceS3); s3Err != ErrNone {
 			return cred, owner, s3Err
 		}
@@ -596,7 +596,7 @@ func isPutActionAllowed(ctx context.Context, atype authType, bucketName, objectN
 	case authTypeSignedV2, authTypePresignedV2:
 		cred, owner, s3Err = getReqAccessKeyV2(r)
 	case authTypeStreamingSigned, authTypePresigned, authTypeSigned:
-		region := globalServerRegion
+		region := globalSite.Region
 		cred, owner, s3Err = getReqAccessKeyV4(r, region, serviceS3)
 	}
 	if s3Err != ErrNone {