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