Currently, the backend minio server uses the same data directory
as the mint test itself, causing `s3 sync` to fail often.
Now `minio` backend will use a different data directory `/data`
instead of `/mint/data`
Simplify the cmd/http package overall by removing
custom plain text v/s tls connection detection, by
migrating to go1.12 and choose minimum version
to be go1.12
Also remove all the vendored deps, since they
are not useful anymore.
