S3 Publisher: Print the download URL for published artifacts

[legal90]

Description

When the S3 uploading is completed, the download URL will be printed to the Omnibus log output. That is useful if S3 is used as a final hosting platform for these artifacts, so operator can get a result URL directly from the omnibus output.

 Example

$ bundle exec omnibus publish s3 my-bucket --acl public --region eu-central-1 "./pkg/*.dmg"
                    [CLI] I | 2018-03-19T09:36:47+01:00 | Using config from 'omnibus.rb'
       [Command::Publish] I | 2018-03-19T09:36:47+01:00 | Using config from 'omnibus.rb'
            [S3Publisher] I | 2018-03-19T09:36:47+01:00 | Starting S3 publisher
            [S3Publisher] I | 2018-03-19T09:36:48+01:00 | Uploading 'myproj-0.1.0+20180315142042-1.dmg'
            [S3Publisher] I | 2018-03-19T09:36:58+01:00 | Uploading is completed. Download URL (public-read): https://my-bucket.s3.eu-central-1.amazonaws.com/current/myproj/0.1.0%2B20180315142042/mac_os_x/10.10/myproj-0.1.0%2B20180315142042-1.dmg
Published 'myproj-0.1.0+20180315142042-1.dmg' for mac_os_x-10.10

Note: Metadata URL is printed to debug log level only, so it's not shown in the example above.

[legal90]

As I can see, Rubocop offenses (in Travis CI build) are not related to this PR and should be fixed in a standalone PR.

[legal90]

I've rebased the branch. But TravisCI and AppVeyor builds still look broken (is not related to this PR)

[legal90]

cc @chef/omnibus-maintainers is anyone interested in this?

[tas50]

Ping @scotthain

[scotthain]

Looks good - let's get it rebased and merged!

[legal90]

Sorry for a delay. I've rebased the branch now

[scotthain]

Thanks @legal90 much appreciated!