Re ;
I've found the reason:
Your openVZ container come with bind9 pre-installed but the init script links are missing.
So, I've do:
Shell-Script
- root@green:/etc/init.d# update-rc.d bind9 defaults
- Adding system startup for /etc/init.d/bind9 ...
- /etc/rc0.d/K20bind9 -> ../init.d/bind9
- /etc/rc1.d/K20bind9 -> ../init.d/bind9
- /etc/rc6.d/K20bind9 -> ../init.d/bind9
- /etc/rc2.d/S20bind9 -> ../init.d/bind9
- /etc/rc3.d/S20bind9 -> ../init.d/bind9
- /etc/rc4.d/S20bind9 -> ../init.d/bind9
- /etc/rc5.d/S20bind9 -> ../init.d/bind9
And now, when I reboot, all is ok 
To resume
The problem is not related to i-MSCP It's a crappy openVZ container image
This mean that each time you install i-MSCP on such crappy container, you must first create the init links for bind9 as shown above.
BTW: It's not related to this thread but you must also purge any sendmail related package.
Finally, can you give a new try (new container, init links creation for bind9, stop sendmail service, purge of any sendmail related package, new i-MSCP installation, reboot) to see if all is ok ?