Wrap "mongod_flags" onto multiple lines for readability

We started doing this in the etc/evergreen.yml project configuration file back in SERVER-31976. We should do the same in the etc/perf.yml project configurration file. See https://stackoverflow.com/questions/3790454/in-yaml-how-do-i-break-a-string-over-multiple-lines/21699210#21699210 to read more about all the other things you can do with YAML strings.