Added add_info_message function, which will be shown on every exit.

In GH-4086, we have to add a info message in case of auto-installing python-apt.
IMHO this could be useful in more modules, so I implemented it in module_common.

Usage:

    module = AnsibleModule
    module.add_info_message('Module is deprecated. Use foobar instead.')

Or as it would be for GH-4086:

    try:
        import apt
        import apt_pkg
    except:
        try:
            module.run_command('apt-get install python-apt -y')
            import apt
            import apt_pkg
            module.add_info_message('python-apt was auto-installed to enable the future operation of this module.')
        except:
            module.fail_json(msg="Could not import python modules: apt, apt_pkg. Please install python-apt package.")

Result installing foobar failed not having python-apt available:

    TASK: [install default packages] ********************************************** 
    failed: [web.example.com] => (item=foobar) => {"failed": true, "item": "foobar"}
    msg: python-apt was auto-installed to enable the future operation of this module. No package matching 'foobar' is available.