Fix to enable negative test image_invalid_metadata

Remove skips in test_images_oneserver

* Remove skip test_create_image_specify_invalid_metadata
* Fix assertRaises bug to match against exceptions.BadRequest instead
  of exceptions.OverLimit which was causing this test case to fail.

Fixes bug 1005423

Change-Id: I877122880085d1fd69abe1d6d5b730701084c4a9
Merge "Fix to enable negative test image_invalid_metadata"