[TASK] Install tool: Refactor db connect configuration settings

Refactor needsExecution() logic to make sure the mandatory settings
(username, password, host, port) are present in LocalConfiguration.php
before trying to establish a database connection using these settings.

This fixes a possible redirect loop if some of the mandatory settings
were not present, but the database connection check still succeeded.

Change-Id: Ifbf98a4bdcb62d9d29b7fbc1f3a03d3a7d621492
Resolves: #51433
Releases: 6.2
Reviewed-on: https://review.typo3.org/23394
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn