Make platform branch configurable.

playbooks/roles/test_build_server/defaults/main.yml

@@ -16,3 +16,4 @@
 #
 test_build_server_user: jenkins
 test_build_server_repo_path: /home/jenkins
+test_edx_platform_version: master

playbooks/roles/test_build_server/tasks/main.yml

@@ -24,7 +24,7 @@
   git: >
     repo=https://github.com/edx/edx-platform.git
     dest={{ test_build_server_repo_path }}/edx-platform-clone
-    version=master
+    version={{ test_edx_platform_version }}
   sudo_user: "{{ test_build_server_user }}"
 
 - name: get xargs limit