dist/common/systemd: run node-exporter.service as scylla user
dist/common/systemd: run node-exporter.service as scylla user

For security reason, we should run node-exporter.service as scylla user,
instead of root.

Fixes #1968

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1483543419-16541-1-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,raphaelsc/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla