Fix HTTP proxy wide settings

+ Set proper file name for HTTP proxy configuration for APT
+ Set HTTP proxy wide settings in /etc/environment
Honor system-wide defined HTTP Proxy in REST client
Merge pull request #721 from Zentyal/ejhernandez/proxy-not-honored

Set system-wide HTTP proxy properly in precise
Pass HTTP_PROXY sys env var

+ Pass HTTP_PROXY system environment variable to CGIs as they are
  used in Zentyal modules
Merge pull request #722 from Zentyal/ejhernandez/proxy-not-honored

Pass HTTP_PROXY sys env var
Merge branch '3.0' into 3.2

Conflicts:
	main/core/ChangeLog
	main/network/ChangeLog
	main/network/src/EBox/Network.pm
	main/remoteservices/ChangeLog