Deprecate the skip_unless_attr decorator.

This decorator was used at only one place (ListServerFiltersTestJSON)
and those tests are skipped in the Gate anyway (SKIPPED: Only one image found)
These tests were poorly written anyway, the resource_setup() method
goes against are principles in [1]: Using discovery for skipping tests
is generally discouraged.

This decorator encourages bad practise, like the usage of class variables.
We should use the generic and well known testtools.SkipIf/Unless decorator
instead.

[1] : https://github.com/openstack/tempest/blob/master/HACKING.rst#skipping-tests

Change-Id: I639f324d5b38cd154b3ecdb89b56ff2ee279c4ff
3 files changed