Test Cloudflare CDN on canary.gitlab.com

Spawned from https://gitlab.com/gitlab-com/infrastructure/issues/57#note_33100110

Documentation on Rail CDN configuration here: http://guides.rubyonrails.org/asset_pipeline.html#cdns

Process:

• Obtain a certificate for cdn-canary.gitlab.com -> https://gitlab.com/gitlab-com/infrastructure/issues/2115
• Using ops-contact+cloudflare@gitlab.com (stored in the production vault) setup a Cloudflare CDN cdn-canary.gitlab.com with backend canary.gitlab.com, limited to /assets/ directory
• Set GITLAB_CDN_HOST to cdn-canary.gitlab.com within Canary web workers
• Test Notify in the development channel that we have this setup so they can test.

@pcarranza my biggest concern is that Cloudflare will probably refuse to use canary.gitlab.com as a backend, since it's TLS configuration is currently wrong: it's using the gitlab.com TLS certs. This is probably something worth fixing if we can people to use canary.gitlab.com. cc @briann