It is possible to install a site without install_profile in settings.php

If you have an existing settings.php with database settings and a hash salt it is possible to install drupal without the install_profile setting being written to settings.php

• If the settings.php exists and has install_profile - it should be used and the form to select the profile should be skipped
• If settings.php exists and is writable and does not have the install_profile - it should be selected using the form and written to settings.php
• If settings.php exists and is not writable and does not have the install_profile - we should get a requirements error

Discovered in #2090115: Don't install a module when its default configuration has unmet dependencies