add rpc_upgrade_mode as config option for tls upgrades

command/agent/config-test-fixtures/basic.hcl

@@ -152,6 +152,7 @@ tls {
     ca_file = "foo"
     cert_file = "bar"
     key_file = "pipe"
+    rpc_upgrade_mode = true
     verify_https_client = true
 }
 sentinel {

command/agent/config_parse.go

@@ -771,6 +771,7 @@ func parseTLSConfig(result **config.TLSConfig, list *ast.ObjectList) error {
 		"http",
 		"rpc",
 		"verify_server_hostname",
+		"rpc_upgrade_mode",
 		"ca_file",
 		"cert_file",
 		"key_file",

command/agent/config_parse_test.go

@@ -172,6 +172,7 @@ func TestConfig_Parse(t *testing.T) {
 					CAFile:               "foo",
 					CertFile:             "bar",
 					KeyFile:              "pipe",
+					RPCUpgradeMode:       true,
 					VerifyHTTPSClient:    true,
 				},
 				HTTPAPIResponseHeaders: map[string]string{