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.

roles/bootstrap-os/tasks/bootstrap-centos.yml

@@ -30,6 +30,7 @@
 - name: Add proxy to /etc/yum.conf if http_proxy is defined
   lineinfile:
     path: "/etc/yum.conf"
+    regexp: "^proxy=.*$"
     line: "proxy={{ http_proxy }}"
     create: true
     state: present