Add os-networks JSON client for tempest

There is no client for the os-networks API.
Scenario tests do use that API, and migrating them to tempest clients
triggered the need for this client to exists.
Scenario tests only run with JSON, so only providing JSON implementation.

Partially implements bp:tempest-client-scenarios

Change-Id: I85da97a98fc558af14e2a9fc019b6e712653ecf2
Deacrease the required image store size by 4 GiB

The test_volumes_actions inreased the volume size by 1 before,
upload the volume to the image store.

This test run with xml/json and with volume v1/v2 api version,
 so it is 4 in total.

This change decreases the theoretical maximum image store size,
 consumed by the test suite in a parallel run.

This change does the followings:
* The extend test cases moved to dedicated file.
* The image deletion now a waited action,
  so the image delteion needs to be comploted before the
  next test.
* Use HEAD instead of get for checking image deletion,
  in order to avoid full image download just for status check.

Change-Id: I982d783312eef5b18dc7518e119b0aa018a195d5
Related-Bug: #1347692
Merge "Deacrease the required image store size by 4 GiB"
Merge "Add os-networks JSON client for tempest"
Merge branch 'master' of https://github.com/openstack/tempest