[AIRFLOW-3089] Drop hard-coded url scheme in google auth redirect. (#3919)
[AIRFLOW-3089] Drop hard-coded url scheme in google auth redirect. (#3919)

The google auth provider hard-codes the `_scheme` in the callback url to
`https` so that airflow generates correct urls when run behind a proxy
that terminates tls. But this means that google auth can't be used when
running without https--for example, during local development. Also,
hard-coding `_scheme` isn't the correct solution to the problem of
running behind a proxy. Instead, the proxy should be configured to set
the `X-Forwarded-Proto` header to `https`; Flask interprets this header
and generates the appropriate callback url without hard-coding the
scheme.
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow