get_device_warnings decided two things for the application: that any
pending update is a warning (it re-read the updates for that), and
whether apt's proxy is the one the application's setting wants, which
it was handed as optional_args["apt_proxy_url"].
The driver reports facts instead (napalm-device-types 4.14):
get_package_proxy() says which proxy apt fetches through, None for none
or on a non-apt host; the updates are get_available_updates() as
before. get_device_warnings and the apt_proxy_url argument go.
BREAKING CHANGE: get_device_warnings and optional_args["apt_proxy_url"]
are removed. Requires napalm-device-types 4.14.
Closes#28
Christian Manivong