Retry acme.sh cloning

Sometimes we observe failures to clone acme.sh from GitHub. Retry it
up to three times with a two-second delay between each try, in hopes
of failing these jobs less often.

While we're here, update the URL to a more current one which doesn't
need redirecting.

Change-Id: I5179c0482afcb407b7d28d4c3ce73d96d41c6493

playbooks/roles/letsencrypt-acme-sh-install/README.rst

@@ -1,6 +1,6 @@
 Install acme.sh client
 
-This makes the `acme.sh <https://github.com/Neilpang/acme.sh>`__
+This makes the `acme.sh <https://github.com/acmesh-official/acme.sh>`__
 client available on the host.
 
 Additionally a ``driver.sh`` script is installed to run the

playbooks/roles/letsencrypt-acme-sh-install/tasks/main.yaml

@@ -1,8 +1,12 @@
 - name: Install acme.sh client
   git:
-    repo: https://github.com/Neilpang/acme.sh
+    repo: https://github.com/acmesh-official/acme.sh
     dest: /opt/acme.sh
     version: dev
+  register: clone_acmesh_result
+  until: clone_acmesh_result is not failed
+  retries: 3
+  delay: 2
 
 - name: Install letsencrypt group
   group: