Minion log file 0600

Set restrictive umask on minion log file during the verify step to ensure it's created with 0600. (It is currently created as 0644). I cannot think of any cases offhand where salt would need to read this log file but not be able to write to it. This change has been approved by @basepi in his comments on the issue but I think @thatch45 should probably be the one to merge this so that he can check for cases that I might not have thought of. Refs #10030.