Add retries for UC fetching over HTTP

In CI we've spotted multiple occurances of connection timeouts towards
Gitea resulting in intermittent failures.

A retry is being added here to reduce risk of intermittent failures.

Change-Id: Ic13fc1d62fae1125661b5ca193f25e2fadd6fdb9

tasks/main.yml

@@ -55,6 +55,9 @@
     url: "{{ rally_upper_constraints_url }}"
     return_content: yes
   register: _u_c_contents_uri
+  until: _u_c_contents_uri is success
+  retries: 5
+  delay: 5
   check_mode: false
   when: rally_openstack_git_repo is search('http')