Merge pull request #4139 from edx/estute/fix-jenkins-github-variables

rename jenkins github plugin variables

playbooks/roles/jenkins_common/templates/config/github_config.yml.j2

 ---
 {% for config in jenkins_common_github_configs %}
-- CREDENTIAL_ID: '{{ config.GITHUB_CREDENTIAL_ID }}'
-  MANAGE_HOOKS: '{{ config.GITHUB_MANAGE_HOOKS }}'
-  USE_CUSTOM_API_URL: '{{ config.GITHUB_USE_CUSTOM_API_URL }}'
-  API_URL: '{{ config.GITHUB_API_URL }}'
-  CACHE_SIZE: '{{ config.GITHUB_CACHE_SIZE}}'
+- CREDENTIAL_ID: '{{ config.CREDENTIAL_ID }}'
+  MANAGE_HOOKS: '{{ config.MANAGE_HOOKS }}'
+  USE_CUSTOM_API_URL: '{{ config.USE_CUSTOM_API_URL }}'
+  API_URL: '{{ config.API_URL }}'
+  CACHE_SIZE: '{{ config.CACHE_SIZE}}'
 {% endfor %}