Add timeout for http.Client

[embano1]

• Invoker http.Client now also uses the user-defined upstream timeout for http requests (before: only net.Dialer used the timeout for initial connection establishing), see https://godoc.org/net/http#Client
• Similar to how OpenFaaS http connection handling uses the timeout in http.Client
• A separate context.Context for the httpReq in invoker's c.Do() is not needed right now as the underlying http.Client now has a timeout for the request (unless there's need for context.Context)

Signed-off-by: Michael Gasch embano1@live.com

[alexellis]

Thank you for this PR 👍 🎉

Could we also add the context in the c.Do? The reason I ask is that the http.Client timeout does not give the correct HTTP status code, it will just disconnect. That's the reason I added what looks like duplication in openfaas/faas/gateway and openfaas-incubator/of-watchdog.