Pass extra mysqld flags to mysql_install_db

[tianon]

Fixes #356

I've tested and verified this with --datadir=/data/db --innodb-log-file-size=1G specified at runtime, and everything works (on both mysql:5.5 and mysql:5.6), with the appropriate and expected output from mysql_install_db while initializing InnoDB:

...
171221 21:35:35  InnoDB: Setting file ./ibdata1 size to 10 MB
InnoDB: Database physically writes the file full: wait...
171221 21:35:35  InnoDB: Log file ./ib_logfile0 did not exist: new to be created
InnoDB: Setting log file ./ib_logfile0 size to 1024 MB
InnoDB: Database physically writes the file full: wait...
InnoDB: Progress in MB: 100 200 300 400 500 600 700 800 900 1000
MySQL init process in progress...
MySQL init process in progress...
171221 21:35:37  InnoDB: Log file ./ib_logfile1 did not exist: new to be created
InnoDB: Setting log file ./ib_logfile1 size to 1024 MB
InnoDB: Database physically writes the file full: wait...
InnoDB: Progress in MB: 100 200 300 400 500 600 700 800 900 1000
MySQL init process in progress...
...

[tianon]

(Used a custom --datadir in my testing to verify that this doesn't break any of that functionality, and I also verified that the containers were in fact writing to /data/db as a result.)