Parameterize URL for lockss-installer's Git repo.

roles/lockss/tasks/main.yml

@@ -119,7 +119,7 @@
 
 - name: Pull LOCKSS repository.
   git:
-    repo: https://github.com/lockss/lockss-installer
+    repo: "{{ lockss_git_url }}"
     dest: /usr/src/lockss
     version: "{{ lockss_git_version }}"
     depth: 1

roles/lockss/vars/main.yml

 ---
 
+lockss_git_url: https://github.com/lockss/lockss-installer
 lockss_access_subnet: 10.0.0.0/8