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 <syuu@scylladb.com>
Message-Id: <1481643873-28984-1-git-send-email-syuu@scylladb.com>

dist/common/scripts/scylla_selinux_setup

@@ -4,7 +4,7 @@
 
 . /usr/lib/scylla/scylla_lib.sh
 
-if is_debian_variant; then
+if ! is_debian_variant; then
     if [ "`sestatus | awk '{print $3}'`" != "disabled" ]; then
         setenforce 0
         sed -e "s/enforcing/disabled/" /etc/sysconfig/selinux > /tmp/selinux