Add ECOMMERCE_SECURE_PROXY_SSL_HEADER

[viadanna]

This PR adds ECOMMERCE_SECURE_PROXY_SSL_HEADER ansible variable and the SECURE_PROXY_SSL_HEADER configuration on ecommerce.yml

Testing

1. Provision a VM for testing (Vagrant is recommended).
2. Run the Ecommerce playbook without any variables to make sure there's a sane default:

$ ansible-playbook -vvv --user=ubuntu -i hosts playbooks/ecommerce.yml

3. Make sure the defaults produce a sane /edx/etc/ecommerce.yml:

(...)
SECURE_PROXY_SSL_HEADER: null
(...)

4. Run the playbook setting a value for the added config:

$ ansible-playbook -vvv --user=ubuntu -i hosts playbooks/ecommerce.yml -e '{"ECOMMERCE_SECURE_PROXY_SSL_HEADER":["HTTP_X_FORWARDED_PROTO","https"]}'

5. Make sure the resulting configuration is correct:

(...)
SECURE_PROXY_SSL_HEADER:
- HTTP_X_FORWARDED_PROTO
- https
(...)

[giovannicimolin]

👍

• I tested this:

1. Provisioned an Ubuntu 16.04 vagrant instance.
2. Set up a ssh-config to ansible connect with the vagrant instance and copied a working ansible.cfg file with the correct settings.
3. Checked out this branch.
4. Run provisioning with this line:

ansible-playbook -v --user=vagrant -i hosts playbooks/ecommerce.yml -e @../configuration-secure/vars-stage.yml -e EDXAPP_LMS_ROOT_URL=123 -e EDXAPP_LMS_BASE_SCHEME=123

4. Checked that the file had the correct default set ✔️

SECURE_PROXY_SSL_HEADER: null

5. Run the provisioning again with this line:

ansible-playbook -v --user=vagrant -i hosts playbooks/ecommerce.yml -e @../configuration-secure/vars-stage.yml -e EDXAPP_LMS_ROOT_URL=123 -e EDXAPP_LMS_BASE_SCHEME=123 -e '{"ECOMMERCE_SECURE_PROXY_SSL_HEADER":["HTTP_X_FORWARDED_PROTO","https"]}'

6. Checked that the file had the correct contents ✔️

SECURE_PROXY_SSL_HEADER:
- HTTP_X_FORWARDED_PROTO
- https

• I read through the code
• I checked for accessibility issues NA
• Includes documentation NA

@viadanna Nice work on this! Good to merge.