Allow overriding of the django SESSION_COOKIE_SECURE setting

https://django.readthedocs.org/en/1.4/ref/settings.html

playbooks/roles/edxapp/defaults/main.yml

@@ -543,6 +543,8 @@ EDXAPP_SOCIAL_AUTH_SAML_SP_PUBLIC_CERT: ''
 # Only set this to true for client side profiling, never for production
 EDXAPP_SESSION_SAVE_EVERY_REQUEST: false
 
+EDXAPP_SESSION_COOKIE_SECURE: false
+
 #-------- Everything below this line is internal to the role ------------
 
 #Use YAML references (& and *) and hash merge <<: to factor out shared settings
@@ -841,6 +843,7 @@ generic_env_config:  &edxapp_generic_env
   EDXMKTG_USER_INFO_COOKIE_NAME: "{{ EDXAPP_EDXMKTG_USER_INFO_COOKIE_NAME }}"
   COMP_THEME_DIR: "{{ EDXAPP_COMP_THEME_DIR }}"
   SESSION_SAVE_EVERY_REQUEST: "{{ EDXAPP_SESSION_SAVE_EVERY_REQUEST }}"
+  SESSION_COOKIE_SECURE: "{{ EDXAPP_SESSION_COOKIE_SECURE }}"
 
 lms_auth_config:
   <<: *edxapp_generic_auth