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/sts-handlers.go

@@ -142,12 +142,12 @@ func checkAssumeRoleAuth(ctx context.Context, r *http.Request) (user auth.Creden
 	default:
 		return user, true, ErrSTSAccessDenied
 	case authTypeSigned:
-		s3Err := isReqAuthenticated(ctx, r, globalServerRegion, serviceSTS)
+		s3Err := isReqAuthenticated(ctx, r, globalSite.Region, serviceSTS)
 		if s3Err != ErrNone {
 			return user, false, STSErrorCode(s3Err)
 		}
 
-		user, _, s3Err = getReqAccessKeyV4(r, globalServerRegion, serviceSTS)
+		user, _, s3Err = getReqAccessKeyV4(r, globalSite.Region, serviceSTS)
 		if s3Err != ErrNone {
 			return user, false, STSErrorCode(s3Err)
 		}