Switch client to JSON-RPC

Modify ipalib.rpc to support JSON-RPC in addition to XML-RPC.
This is done by subclassing and extending xmlrpclib, because
our existing code relies on xmlrpclib internals.

The URI to use is given in the new jsonrpc_uri env variable. When
it is not given, it is generated from xmlrpc_uri by replacing
/xml with /json.

The rpc_json_uri env variable existed before, but was unused,
undocumented and not set the install scripts.
This patch removes it in favor of jsonrpc_uri (for consistency
with xmlrpc_uri).

Add the rpc_protocol env variable to control the protocol
IPA uses. rpc_protocol defaults to 'jsonrpc', but may be changed
to 'xmlrpc'.
Make backend.Executioner and tests use the backend specified by
rpc_protocol.

For compatibility with unwrap_xml, decoding JSON now gives tuples
instead of lists.

Design: http://freeipa.org/page/V3/JSON-RPC
Ticket: https://fedorahosted.org/freeipa/ticket/3299

doc/examples/python-api.py

@@ -34,14 +34,14 @@ api.bootstrap_with_global_options(context='example')
 api.finalize()
 
 # You will need to create a connection.  If you're in_server, call
-# Backend.ldap.connect(), otherwise Backend.xmlclient.connect().
+# Backend.ldap.connect(), otherwise Backend.rpcclient.connect().
 
 if api.env.in_server:
     api.Backend.ldap2.connect(
         ccache=api.Backend.krb.default_ccname()
      )
 else:
-    api.Backend.xmlclient.connect()
+    api.Backend.rpcclient.connect()
 
 
 # Now that you're connected, you can make calls to api.Command.whatever():