Create ~/.my.cnf with root user/password

50-mysql-users and 60-mysql-secure depend on ~/.my.cnf
existing with the correct root password.
This needs to be created as part of the install so that
os-refresh-config will work post install.
On a re-image this may not exist so try with no password
or with /mnt/state/root/metadata.my.cnf
Change-Id: If984c45b5745997a2ec63d9c90d4cf4c105964d2
Merge "Create ~/.my.cnf with root user/password"