Support logging configuration in OpenContrail

Presently much of the logging configuration in both C++ modules and Python modules
are hard coded. This blueprint proposes the mechanism to support logging configuration.

Python modules
---------------
This blueprint proposes
 1. to provide the modular mechanism to use other third party logging libraries like oslo.log or to define new ones.
 2. Enhance the sandesh logger to reading the logging parameters (like handlers, formatters etc) from the configuration file

The below approach can be taken to implement the suggested features.

  a. A base sandesh logger (SandeshBaseLogger) class needs to be defined
  b. sandesh_logger.SandeshLogger should be derived from SandeshBaseLogger
  c. External logging library should derive from SandeshBaseLogger class.
  d. External logging library needs to be outside the OpenContrail source tree and can be a pip package.
  d. The class name of the thirty party logging libraries has to be defined in a configuration file (eg. contrail-api.conf)
  e. class name needs to be passed to sandesh_base.Sandesh object
  f. Sandesh object needs to call a factory method (eg. SandeshBaseLogger.create_logger()) to create the logger class object.
  g. If external class name is not defined in the configuration file, then sandesh_base.Sandesh should use sandesh_logger.SandeshLogger class.
  h. Enhance the sandesh_logger.SandeshLogger to support the configuration function - logging.config.fileConfig()

C++ modules
------------
The logging module needs to be enhanced to read the logging parameters like log handlers, file size, enable syslog etc from configuration files.

Draft code is available here
---------------------------------
https://github.com/enovance/contrail-controller/commit/9b5a4909d7658b9c1d2a80fef6350d2d03ba2f3d

https://github.com/enovance/contrail-sandesh/commit/6d7477ac145b205ebbb59c65761ae605062a2ccc