'Retrieves the specified configuration from the device
This method will retrieve the configuration specified by source and
return it to the caller as a string.  Subsequent calls to this method
will retrieve a new configuration from the device
:args:
arg[0] source: Datastore from which configuration should be retrieved eg: running/candidate/startup. (optional)
default is running.
arg[1] format: Output format in which configuration is retrieved
Note: Specified datastore should be supported by remote device.
:kwargs:
Keywords supported
:command: the command string to execute
:source: Datastore from which configuration should be retrieved
:format: Output format in which configuration is retrieved
:returns: Returns output received from remote device as byte string'

@abstractmethod
def get_config(self, source='running', format='text'):

    pass
