Revert removal of --all-namespaces from policy checks

[rgooler]

#1516 included an undocumented change, removing the --all-namespaces argument from conftest runs.

This is a particularly painful change for myself and others, as it breaks the ability to use namespacing to manage and organize policy collections. Its a significant behavior regression from the beta as well - with no explanation as to why this was desired, or should have been a part of a version bump for conftest.

This PR reverts the parts of #1516 that removed --all-namespaces, and restores the original behavior.

[nishkrishnan]

I just want to close the thread out on this:
#1516 (comment)

Given that this got changed as a part of an upgrade from a pre-release to a production release. I'd rather just extend the policy checking step to support extra args through the server side repo config.

Can get this done and released as part of a minor version bump soon, but the workaround suggested in the the linked issue is good enough to get this working.

[nishkrishnan]

Btw here's the PR: #1543