It is optional to set "view_hints_directory", so we shouldn't insist that
it is defined in scylla.yaml on upgrade.
Fixes #4091.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
Message-Id: <20190114125225.10794-1-nyh@scylladb.com>
(cherry picked from commit 9062750089)