Skip the installation profile installer step if there's only one visible choice

Related: #1186600: Hide the minimal profile so it's only available to advanced users

If that patch goes in, then there's only one visible profile to pick from at install time: standard.

However, the installer will happily show you a silly interstitial screen with one choice on it and force you to click through it. ;) We should skip this stage of the installer altogether if there's only one choice.

Seems like behaviour we could backport to D7, too.