Resolve "Circular dependency detected"

What does this MR do?

Fixes the following error:

$ bundle exec rake gitlab:assets:clean
D, [2017-03-20T16:29:07.247786 #1691] DEBUG -- sentry: ** [Raven] Circular dependency detected: TOP => gitlab:assets:clean => gitlab:assets:clean excluded from capture due to environment or should_capture callback
rake aborted!
Circular dependency detected: TOP => gitlab:assets:clean => gitlab:assets:clean

This fixes the gitlab:assets:clean task. Because it depends on assets:clean, it needs to be told to use the root namespace otherwise it thinks it is trying to call itself.

See: https://github.com/jimweirich/rake/issues/274

This regression was introduced in e6529879 and exists in the 9-0-stable branch, but is not a problem in 8-17-stable. This should be Pick into Stable for 9.0

Are there points in the code the reviewer needs to double check?

Ensure gitlab:assets:clean runs without throwing the error above.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28618 (closed)