Move cache logging to debug

Can get a tad noisy with several tasks all using setting up multiple caches.

Testing Done: https://travis-ci.org/pantsbuild/pants/builds/40392152

Bugs closed: 783

Reviewed at https://rbcommons.com/s/twitter/r/1310/
Sync of https://github.com/pantsbuild/pants/commit/205dbd965eaad08d2fcf06112b0322d767c58883