update installation instruction for centos7

these instructions need to be updated:
http://www.observium.org/docs/rhel/

1. Repositories
rpm -Uvh http://pkgs.repoforge.org/rpmforge-release/rpmforge-release-0.5.3-1.el7.rf.x86_64.rpm
rpm -Uvh http://download.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm
(i386 repositories does not apply for centos7)

2. Packages
replace 'php-posix.x86_64' with 'php-posix'
replace 'vixie-cron.x86_64' with 'cronie.x86_64'
replace 'mysql-server.x86_64' with 'mariadb-server.x86_64'
replace 'mysql.x86_64' with 'mariadb.x86_64'
replace 'libvirt' with 'libvirt.x86_64'
add an update command: 'yum update'

3. MySQL Database
add command to set root password: /usr/bin/mysqladmin -u root password 'mysqlpass'
add 'exit' command to get out of mysql

4. Configuration (this part had me confused, and not only me it seems like, please adjust!)
replace 'Edit config.php' with 'Edit /opt/observium/config.php'
replace 'You must change the MySQL username and password' with 'You must change the db_user and db_pass, then use the same account below when you grant all privileges to the database'

5. System
remove the instruction 'mkdir logs' since it is repeated further down
replace 'change /etc/httpd/conf/httpd.conf default to:' with 'open /etc/httpd/conf/httpd.conf and add the following:'

6. Cron
the command '/etc/init.d/cron reload' did not work, i had to use the following steps:

run 'crontab -e'
press 'i' for insert and paste the following text:
33 */6 * * * /opt/observium/discovery.php -h all >> /dev/null 2>&1
*/5 * * * * /opt/observium/discovery.php -h new >> /dev/null 2>&1
*/5 * * * * /opt/observium/poller-wrapper.py 15 >> /dev/null 2>&1
press 'esc', type ':x' to close and save

7. Final Points
replace:
chkconfig mysqld on
chkconfig httpd on
with:
systemctl enable mariadb
systemctl enable httpd
systemctl start mariadb
systemctl start httpd

add steps to open firewall:
firewall-cmd --permanent --zone=public --add-service=http
firewall-cmd --permanent --zone=public --add-service=https
firewall-cmd --reload