remove unused code in serverprovider and tests

This patch removes some unused code in provider.py and test_provider.py.
Specifically in provider.py it removes
class ImageDTO(utils.ImmutableMixin)
in class ProviderFactor it removes functions upload_image and destroy_image

In test_provider.py it removes
class ImageDTOTestCase(test.TestCase)

Change-Id: I4f66ade949b196ace42f0500b4ac3e610e4dbb53