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.

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

java/google/registry/tools/GetKeyringSecretCommand.java

@@ -18,7 +18,6 @@ import com.beust.jcommander.Parameter;
 import com.beust.jcommander.Parameters;
 import google.registry.keyring.api.KeySerializer;
 import google.registry.keyring.api.Keyring;
-import google.registry.tools.Command.RemoteApiCommand;
 import google.registry.tools.params.KeyringKeyName;
 import google.registry.tools.params.PathParameter;
 import java.io.FileOutputStream;