Use os.path.join as possible

This commit makes to use os.path.join instead of using the Unix path
character '/'. This change is not mandatory since we don't support
Windows environments as Tempest execution and, it seems Windows supports
'/' as a path separator. However, using both os.path.join and '/' is a
bit awkward.

Change-Id: I117acb281c352179d526808009e761335bb314fc
6 files changed