untar: ignore '/' in tarball

[yuchen0cc]

What this PR does / why we need it:
It happens that '/' (root dir) exists in some image tarball layer. It will cause userspace convertor failed.
Now, '/' is ignored.
Add a test case for clean_name().

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

[WaberZhuang]

LGTM