Did you do the /usr/bin/mysql_secure_install
Sounds like that is the issue, run mysql_secure_install
It will setup MySQL securely with a simple set of questions and answers.. Yes is the right answer for most of the questions.
Here is a link to LAMP Stack for Centos, the MySQL section is just below the Apache install section..
https://www.digitalocean.com/community/tutorials/how-to-install-linux-apache...
Also good for CentOS 7
john