My experience with the installation of Openproject 8 (community edition) on CentOS 7 wasn't easy as documented.
Certainly not when I am using MariaDB 10.1. 

Add the Openproject  package source:
 sudo wget -O /etc/yum.repos.d/openproject-ce.repo   https://dl.packager.io/srv/opf/openproject-ce/stable/8/installer/el/7.repo

Install the openproject community edition:
sudo yum -y install openproject

Download the new version of Pandoc and unpack it
wget https://github.com/jgm/pandoc/releases/download/2.2.3.2/pandoc-2.2.3.2-linux.tar.gz
cp pandoc-2.2.3.2-linux.tar.gz /opt/openproject/vendor/.
cd /opt/openproject/vendor
tar xzvf pandoc-2.2.3.2-linux.tar.gz
mv pandoc-2.2.3.2 pandoc

Setup account and database on your MariaDB server
mysql -u root -p
DROP DATABASE IF EXISTS openproject;
CREATE DATABASE openproject;

DROP USER IF EXISTS openproject;
CREATE USER 'openproject'@'localhost' IDENTIFIED by 'dbsecret';
GRANT ALL PRIVILEGES ON openproject.* to 'openproject'@'localhost';
FLUSH PRIVILEGES;
SHOW GRANTS FOR 'openproject'@localhost; 
EXIT;

Set specific config settings into your openproject config.
openproject config:set APP_WIZARDS="mysql,apache2,repositories,smtp,openproject"
openproject config:set DATABASE_URL="mysql2://openproject:dbsecret@127.0.0.1:3306/openproject"

Install openproject
openproject reconfigure

mysql/autoinstall                  --> skip     Skip
server/autoinstall                  --> install Install apache2 server
server/hostname                   hostname-server.example.net
server/server_path_prefix     /openproject
server/ssl                                 --> yes     Yes
server/ssl_cert                         /etc/pki/tls/certs/hostname-server.crt
server/ssl_key                         /etc/pki/tls/private/hostname-ser
_server/ssl_ca                              _
_repositories/api-key                  <_press enter>
svn-install                               --> skip     Skip
repositories/git-install           --> skip     Skip
smtp/autoinstall                     --> skip     Skip

After the installation process you notice that you can't browse to URL example
https://hostname-server.example.net/openproject

If you run "netstat -atun" then you see that port 80 is on state LISTEN but you don't find port 443.
The file "/etc/httpd/conf.d/openproject.conf" is missing the line "Listen 443 https"

You have 2 possible solutions:

1.  Add "Listen 443 https" on line 1 of file /etc/httpd/conf.d/openproject.conf
        and reboot the httpd service "systemctl restart httpd"
2.  Add "Listen 443 https" on line 1 of the file /usr/share/openproject/installer/addons/apache2/conf/web-ssl
        and re-run the openproject installation "openproject configure". 

I rather prefer that the developers fix the file "/usr/share/openproject/installer/addons/apache2/conf/web-ssl"

I hope that this will help others that are using CentOS7.

Cheers

This is useful information, thanks! One question, I see you use 'dbsecret' for user 'openproject' in mysql db. are those real or just samples? Also, I see you skipped installing mysql. How do you get the empty DB you configured earlier to be populated?

User "openproject" with password "dbsecret" in mysql db is a sample like the hostname of my server "hostname-server.example.net"
If you select "Skip" under mysql/autoinstall then it still populates the database "openproject" (in my case) with tables and data.
If you select "Install" or "Reuse" then it wants to install mysql package on the current server which is a conflict with MariaDB package.