scylla_util.py: make view_hints_directory setting optional
scylla_util.py: make view_hints_directory setting optional

It is optional to set "view_hints_directory", so we shouldn't insist that
it is defined in scylla.yaml on upgrade.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20190114125225.10794-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla