Implement suggestions from shellcheck

See merge request !4

seperate config from script

See merge request !3

Fix deletion on server

See merge request !2

Previously random files were deleted on the ftp remote because the
requested list of files was not sorted. A simple sort by date is enough
in this case because the directories are already named according to the
date.

lftp: Use specific key file

See merge request !1

idea by SammysHP

lftp was too verbose
the ftpfilesdelete array had magically some ' on first and last element