Openstack horizon returns Danger there was an error submitting the form when add users to projects

Issue

• When try to add user to project, the user will be added to the project.
• But horizon will hang for a minute then display an error
  Danger: there was an error submitting the form
• This can happen because there are too many groups for horizon to process in 1 minute and the default timeout for httpd is 60 seconds