[usecase-webapi] Improve FileSystem usecase

- Delete the exist file before creating the file again

Impacted tests(approved): new 0, update 1, delete 0
Unit test platform: [Tizen IVI]
Unit test result summary: pass 1, fail 0, block 0
Merge pull request #659 from yunxliu/yunxliu/misc/filesystem_update

[usecase-webapi] Improve FileSystem usecase