Configuration format changes for the control-node

control-node now take configuration in the standard ini format.
Default configuration file is read off /etc/contrail/control-node.conf

One can override the values from the config file through command line option
as well. e.g. --DEFAULT.log_level=DEBUG

Use --help to see various options available. contrail-control ackage also
places the default config file under /etc/contrail/control-node.conf

When changes are made to the configuration file, the process must be _restarted_
to take effect. (service supervisord-control restart).
Let defaults come from option parser's default value
Code cleanup
Merge remote-tracking branch 'github/master' into control_node_config_ini
No need to default to empty strings
Add unit tests to test control-node command line options
Add some comments
Merge pull request #393 from Juniper/control_node_config_ini

Configuration format changes for the control-node