Solve IOS-XR admin mode deprecation on certain platforms

[packettoobig]

Pre-Request Checklist

• [ ] Passes rubocop code analysis (try rubocop --auto-correct)
• [ ] Tests added or adapted (try rake test)
• [ ] Changes are reflected in the documentation
• [ ] User-visible changes appended to CHANGELOG.md

Description

Admin mode is being deprecated on some IOS-XR flavors. On others, admin show requires more permissions for the same (or similar) output.

I went for the show inventory all instead of show inventory according to zagosch's comment on the ASR9k platform, even though I did not see any output difference on NCS or C8000 series.

This PR :

• Solves #3166
• Reverts https://github.com/ytti/oxidized/pull/2915