Allow AppEngineConnection to target services other than "tools"

This change required several things:
- Separating out the interfaces that merely do HTTP calls to the backend from those
  that require the remote API (only load the remote API for the latter).  Only the
  tools service provides the remote api endpoint.
- Removing the XSRF token as an authentication mechanism (with OAUTH, we no longer
  need this, and trying to provide it requires initialization of the datastore
  code which requires the remote API)

I can't think of a compelling unit test for this beyond what already exists.
Tested:
  Verified that:
  - nomulus tool commands (e.g. "list_tlds") work against the tools service as they
    currently do
  - The "curl" command hits endpoints on "tools" by default.
  - We can use --server to specify endpoints on the default service.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211510454

java/google/registry/tools/RemoteApiCommand.java

@@ -0,0 +1,23 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/**
+ * Marker interface for commands that use the remote api.
+ *
+ * <p>Just implementing this is sufficient to use the remote api; {@link RegistryTool} will
+ * install it as needed.
+ */
+package google.registry.tools;
+
+interface RemoteApiCommand extends Command {}