Merge branch 'stable-3.1' of git.ispconfig.org:ispconfig/ispconfig3 into stable-3.1

helper_scripts/mydns_to_powerdns_migration.php

install/dist/lib/centos_base.lib.php

install/lib/install.lib.php

interface/web/mail/xmpp_domain_edit.php

server/conf/nginx_vhost.conf.master

server/conf/php-fcgi-starter.master

server/conf/vhost.conf.master

server/plugins-available/apache2_plugin.inc.php

server/plugins-available/mysql_clientdb_plugin.inc.php