#I've tried this with and without the "seen" lines

sudo su

echo "mysql-server-5.5 mysql-server/root_password password " | debconf-set-selections
echo "mysql-server-5.5 mysql-server/root_password seen " | debconf-set-selections
echo "mysql-server-5.5 mysql-server/root_password_again password " | debconf-set-selections
echo "mysql-server-5.5 mysql-server/root_password_again seen " | debconf-set-selections

apt-get -y install mysql-server