add __str__ function to RestClient class

Most tempest tests are based on restfull http call.
Enable to easily explore the state of client object
 would be good for trouble shooting.

Thus, I would suggest to implement __str__ function for RestClient class.

 Closes-Bug: #1227474

Change-Id: If06dfc648a7458133e4f196bdca299f9531738b1
Merge "add __str__ function to RestClient class"