common.sh: Added messages for proxy use and missing information case

The script will check if there's any proxy configuration and, if so, it will
print a message to log file. Besides that, it'll also check if the system
variable "http_proxy" has a value, thus logging "something is wrong".

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>