Working around D8 install checks for writable folders

Hi Torrance,
The install routine in civicrm-drupal checks if certain folders are writeable. AFAIK it needs to do this principally so that it can be sure it can write out a civicrm settings file.

Platform.sh (and possibly pantheon as well?) use a read only file system, so in this host this check has to be patched around to get the installation working. On platform.sh the civicrm settings file (like the drupal settings file) has to be pre-created, and copied into place during deployment while the filesystem is still writeable. (whereas installation is technically post deployment).

Proposed resolution: change the check from "is the folder writeable" to "is the folder is writeable OR is the settings file already in the folder".

If you're OK with this resolution, I'll get my Drupal developers to submit a pull request.

Jonathan