Permission denied:proxy: HTTP :: This error is caused when you try to use proxy balancer when seLinux is enabled. The value for httpd_can_network_connect is turned off by default. You can work around this problem as explained below.
See the selinux status by issusing the command below :
sestatus -b | grep httpd
Now change the value to enabled by issuing the following command :
setsebool -P httpd_can_network_connect=1