Add krbticketPolicyAux objectclass if needed

When modifying ticket flags add the objectclass to the object if it is missing.

https://fedorahosted.org/freeipa/ticket/3901
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
Make jsonserver_kerb start a cookie-based session

Part of the work for: https://fedorahosted.org/freeipa/ticket/3299
Add server/protocol type to rpcserver logs

Add the server class name, such as [xmlserver] or [jsonserver_kerb] to
the server logs. This will allow easier debugging of problems specific
to a protocol or server class.