salt-minion started via upstart is not using /etc/environment to
source variables therefore is not using system wide http_proxy parameters if
they are set there. With this patch if salt.minion.env_vars is set to engine:
file salt will set envvars in /etc/default/salt-minion and will use them for
all commands executed remotely via salt. Since /etc/default is not used on
systems with systemd override for salt-minion to use /etc/environmnet will be
set.

Change-Id: Icfe122de2486a83820a0bd2e2aeea4be3796c6aa
Closes-bug: PROD-16676