Brew services has installed this plist file for mysql.
~ $ brew services list
Name Status User Plist
mysql56 started mh /Users/mh/Library/LaunchAgents/homebrew.mxcl.mysql56.plist
.plist files that come with Formulas can be found in Cellar dir. For instance
homebrew.mxcl.mysql.plist is in
/usr/local/Cellar/mysql/5.7.14. You can modify it and
brew services will use it.
Alternatively you can modify
/Library/LaunchDaemons/homebrew.mxcl.mysql.plist and make it immutable with
sudo chflags uchg /Library/LaunchDaemons/homebrew.mxcl.mysql.plist. This won't allow
brew to rewrite or modify