Remove /tmp and /var/log from reserved paths (#14686)

[jacksgt]

Fixes #14686

Proposed Changes

As discussed in #14686, the runtime contract REQUIRES /tmp to be a read-writeable folder and /var/log MAY BE writeable.
Since this can be achieved with many different means users should be able to mount volumes at these locations (e.g. /tmp for sharing data between containers, /var/log for shipping logs with a sidecar etc.)

/tmp MUST be Read-write. SHOULD be backed by tmpfs if disk load is a concern.
/var/log MAY be a directory with write permissions for logs storage. Implementations MAY permit the creation of additional subdirectories and log rotation and renaming.

https://github.com/knative/specs/blob/a8e7926c041baa58366f4939df9f3fdc866dbbb5/specs/serving/runtime-contract.md#default-filesystems

Release Note

Remove /tmp and /var/log from reserved paths

[knative-prow]

Welcome @jacksgt! It looks like this is your first PR to knative/serving 🎉

[knative-prow]

Hi @jacksgt. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dprotaso]

/ok-to-test

[dprotaso]

/lgtm
/approve

[dprotaso]

Hey thanks for the PR - you'll want to update the tests

[jacksgt]

@dprotaso Thanks for the hint, I adjusted the tests. CI passed now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (514e466) 86.08% compared to head (01a70b2) 86.00%.
Report is 39 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14719      +/-   ##
==========================================
- Coverage   86.08%   86.00%   -0.08%     
==========================================
  Files         197      197              
  Lines       14937    14950      +13     
==========================================
  Hits        12858    12858              
- Misses       1770     1780      +10     
- Partials      309      312       +3     

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[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, jacksgt

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