Summary:
Use ```FileSystem::Default``` to read ```/proc/sys/kernel/uuid```, so it works for ```Envs``` with remote ```FileSystem``` as well.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7672
Reviewed By: riversand963
Differential Revision: D24998702
Pulled By: anand1976
fbshipit-source-id: fa95c1d70f0e4ed17561201f047aa055046d06c3