[BUGFIX] Heal exception in install tool if config value is missing

Upon missing SYS/isInitialDatabaseImportDone configuration value, the
install tool throws an exception.
Merging the LocalConfiguration array with the DefaultConfiguration
solves the problem.

Change-Id: Ic11e7ae4d4b38e50bf8b1cbd6f4ba649ec2edffa
Resolves: #69777
Relates: #51930
Releases: master
Reviewed-on: http://review.typo3.org/43290


Reviewed-by: Stefan Froemken <froemken@gmail.com>
Tested-by: Stefan Froemken <froemken@gmail.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>