errors in kea-admin script and related scripts

Reported via a Kea support customer:

Found annoying error in kea-admin, the $prefix environment variable is set but not exported so it cannot be used by scripts in $prefix/share/kea/scripts/mysql/*.sh.

Also there are errors in prefix/share/kea/scripts/admin-utils.sh at lines 25 and 39, where the --host="{db_host}" parameter is missing so the mysql commands are always attempted towards the local database even if -h or --host parameter is used in kea-admin calls.

I suspect the same problems could be in other backends as well but I didn't check them.

He attached his proposed corrections to admin-utils.sh and kea-admin.