Fix nginx start.sh script, to set right permissions for dump directory

Run `chmod -R 755 /var/www/nailgun/*`, to
set permissions for /var/www/nailgun/dump
which is symlink to /dump. We have to do it,
because chmod -R doesn't follow symlinks.

Change-Id: I43f9b9164d8f0f4d8ddb7d06cc2549145bd135bb
Closes-bug: #1333631