net: http: allow HTTP_NETWORK_TIMEOUT to be configured

Currently, the HTTP_NETWORK_TIMEOUT setting is hard-coded as 20 seconds.
Not every application may want to wait that long, so let's change this
to a CONFIG option: CONFIG_HTTP_CLIENT_NETWORK_TIMEOUT

NOTE: This also removes HTTP_NETWORK_TIMEOUT from the public http.h
include file.  It was not being used externally to HTTP client sources.

Signed-off-by: Michael Scott <michael.scott@linaro.org>