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"