Response extension support for openstackcli common

* Supports adding deserialization support for response properties added
  by optional api 'request extensions' as plugins to the openstackcli
  base response deserializers.
* Made openstackcli behaviors extensible by default.
* Added some helper methods to openstackcli client.
* Changed name of _dict_to_metadata_cmd_string method to
  _dict_to_string
* Added base openstackcli behaviors

Change-Id: I4ed8cdbf888385e8798b292f1932d3f68394f292