After merging redis sentinel patches+fixes, add an option to disable overwriting the original file

[Tyson Andre]

(sentinel patches will overwrite the list of leader redis servers to send commands to)

• Many users may deploy config files owned by another user (e.g. readonly file owned by root) or want to preserve comments, etc
• The sentinel patches don't escape all strings - rare cases may cause problems
• twemproxy restarts should be infrequent