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_test.go

@@ -397,7 +397,7 @@ func TestIsReqAuthenticated(t *testing.T) {
 
 	// Validates all testcases.
 	for i, testCase := range testCases {
-		s3Error := isReqAuthenticated(ctx, testCase.req, globalServerRegion, serviceS3)
+		s3Error := isReqAuthenticated(ctx, testCase.req, globalSite.Region, serviceS3)
 		if s3Error != testCase.s3Error {
 			if _, err := ioutil.ReadAll(testCase.req.Body); toAPIErrorCode(ctx, err) != testCase.s3Error {
 				t.Fatalf("Test %d: Unexpected S3 error: want %d - got %d (got after reading request %s)", i, testCase.s3Error, s3Error, toAPIError(ctx, err).Code)
@@ -435,7 +435,7 @@ func TestCheckAdminRequestAuthType(t *testing.T) {
 	}
 	ctx := context.Background()
 	for i, testCase := range testCases {
-		if _, s3Error := checkAdminRequestAuth(ctx, testCase.Request, iampolicy.AllAdminActions, globalServerRegion); s3Error != testCase.ErrCode {
+		if _, s3Error := checkAdminRequestAuth(ctx, testCase.Request, iampolicy.AllAdminActions, globalSite.Region); s3Error != testCase.ErrCode {
 			t.Errorf("Test %d: Unexpected s3error returned wanted %d, got %d", i, testCase.ErrCode, s3Error)
 		}
 	}