Failover for adding proxy when line exists in file (#5751)

The 'regexp' parameter matches last occurrence of a line starting with 'proxy=' and replaces it with the one defined in 'line' parameter. If no match - it works same way as before. This fixes resuming cluster deployments failed after that task (if there was no more than one line starting with 'proxy' in the yum.conf file - this condition should also be reassured with the change introduced here) eg. if they were initiated with Terraform.