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

ipatests/test_cmdline/test_cli.py

@@ -25,8 +25,8 @@ class TestCLIParsing(object):
 
     def run_command(self, command_name, **kw):
         """Run a command on the server"""
-        if not api.Backend.xmlclient.isconnected():
-            api.Backend.xmlclient.connect(fallback=False)
+        if not api.Backend.rpcclient.isconnected():
+            api.Backend.rpcclient.connect(fallback=False)
         try:
             api.Command[command_name](**kw)
         except errors.NetworkError: