Need to make assert_that(stderr, is_(empty())) more smart

[Anastasia Kuznetsova]

During running cli tests on node with newer clients than in requirements.txt some tests failed with following errors:

if check:
> assert_that(stderr, is_(empty()))
E AssertionError:
E Expected: an empty collection
E but: was 'WARNING (shell) "heat stack-delete" is deprecated, please use "openstack stack delete" instead'

Clients are newer thus some calls are deprecated.
We will face these fails when clients in requirements are updated, so need to rewrite such assert_that now to ignore such warnings in future.