Make rest_client module py34-compatible

We are testing tempest-lib on py27 and py34 now.
To move rest_client module to tempest-lib, this patch makes it
py34-compatible.

* iteritems() has been removed since py3.
* items() returns dict_keys object on py3, so it is necessary to
  convert the return value with list() clearly to avoid TypeError.

Change-Id: Ie9105b5d01e7883213c1d3398cc5fe56782920d9
2 files changed