fix: allow mfe_deployer to run
The `mfe` role requires `COMMON_ECOMMERCE_BASE_URL` to be set, but that variable is not defined in `common_vars`, `ecommerce`, or anywhere else.
Instead of introducing possibly breaking changes in the `ecommerce` or `mfe` roles, simply set it to the default `ECOMMERCE_ECOMMERCE_URL_ROOT` here so that the native playbook can complete successfully.
Configuration Pull Request
Make sure that the following steps are done before merging:
[ ] A DevOps team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
[ ] Are you adding any new default values that need to be overridden when this change goes live? If so:
[ ] Update the appropriate internal repo (be sure to update for all our environments)
[ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.
[ ] Add an entry to the CHANGELOG.
[ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
[ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?