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

ipaserver/advise/base.py

@@ -162,9 +162,9 @@ class IpaAdvise(admintool.AdminTool):
         advice.set_options(self.options)
 
         # Print out the actual advice
-        api.Backend.xmlclient.connect()
+        api.Backend.rpcclient.connect()
         advice.get_info()
-        api.Backend.xmlclient.disconnect()
+        api.Backend.rpcclient.disconnect()
         for line in advice.log.content:
             print line