test_s3: use try_import from common

Change-Id: I3055c3d2f8061b91b038286d8ff2c5cbdcd41efd
Add missing logging import

Change-Id: I863a4b69fe4d050165d304bbf630c57ce171b1bc
Use yaml.safe_load: full yaml.load isn't needed

The only reason to use yaml.load instead of safe_load is if one wants to
load serialized objects. Heat's use case is purely to load basic data
structures such as maps/lists/strings. Fixes bug #1117820

Change-Id: I4f6cf2ed4e15405f8b296ccaec737a3779c9867d
Merge "test_s3: use try_import from common"
Merge "Add missing logging import"