dist/common/scripts/scylla_selinux_setup: correct CentOS/RHEL detection
dist/common/scripts/scylla_selinux_setup: correct CentOS/RHEL detection

CentOS/RHEL is using SELinux, and it's NOT Debian variant, so fixed from
"is_debian_variant" to "! is_debian_variant".

Fixes #1930

Signed-off-by: Takuya ASADA <4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>
Message-Id: <1481643873-28984-1-git-send-email-4ebfb6f510af412d6e89fefa3ab96d9c8b5e3fff@scylladb.com>

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