If HTTP[S]_PROXY is defined, honor it in python requests usage
as well as pass it on to skopeo.
If http[s]_proxy is defined in atomic.conf, use it; however, environment
variables will override these if defined.
Added --insecure to Atomic push so the user can override the logic
(or lack thereof) around deducing if a registry is insecure. Also
needed for integration tests.
Closes: #964
Approved by: rhatdan