Verify site deletes external database info in settings.php

When verifying a site, Aegir rewrites settings.php which causes settings for external databases to be lost.
If this cannot be changed, it would be helpful if database acess info coud be expressed as an array.

Currently, in settings.php for sites created using Aegir, the database access is expressed as:

$db_url = 'mysqli://site_000:password@localhost/site_000';

If that were changed to:

$db_url['default'] = 'mysqli://site_000:password@localhost/site_000';

$db_url would be converted to an array. Doing so would allow additional external databases to
be set by modifying the $db_url array. This could add a database connection in a module:

$db_url['custom'] = 'mysqli://custom_user:password@localhost/custom';

Converting $db_url to an array in settings.php as described would not cause a problem for those who do not need to access external databases and would ease the problem caused by rewriting the settings.php file on
verify for those who do.