OwnCloud 9.1 on CentOS 7. I wanted to configure External Storage to access an SFTP file share. It was failing with "permission denied" and "unable to connect". I traced the reason to one of the SELinux booleans. The "httpd_can_network_connect" boolean is default to "off". The following command turns it on, and sets it to persist across reboots:
setsebool -P httpd_can_network_connect on
This is the only thing I had to change from CentOS default to get SFTP External Storage working. Hope this helps someone else searching on this issue.