Allow running prospectus playbook in parts

[jdmulloy]

By default the playbook should run in order as normal. This change provides 3 new variables which allows a build pipeline to run the prospectus build in 3 parts.

1. PROSPECTUS_ENABLE_PRE_BUILD: Runs everything before npm run build
2. PROSPECTUS_ENABLE_NPM_BUILD: Runs npm run build
3. PROSPECTUS_ENABLE_PRE_BUILD: Runs everything after npm run build

This will allow us to run npm run build directly via ssh so we can see error output without waiting for it to finish and return to ansible.

This will also allow time for the nginx build to run in parallel as soon as the redirects file has been generated.

Configuration Pull Request

Make sure that the following steps are done before merging:

• [ ] A SRE 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 SRE 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? 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?