dist: migrate gentoo variant setup scripts from /sbin/service to /sbin/rc-service

the 'service' binary has been removed from gentoo as per news 2017-10-13:
https://gitweb.gentoo.org/data/gentoo-news.git/plain/2017-10-13-openrc-service-binary-removal/2017-10-13-openrc-service-binary-removal.en.txt

this patch updates the scylla setup related scripts where it was used and
make use of the 'rc-service' binary instead

Signed-off-by: Alexys Jacob <ultrabug@gentoo.org>
Message-Id: <20180119161310.15435-1-ultrabug@gentoo.org>

dist/common/scripts/node_exporter_install

@@ -35,7 +35,7 @@ if is_gentoo_variant; then
        echo "net-analyzer/prometheus-node_exporter does not install systemd service files, please fill a bug if you need them."
     else
         rc-update add prometheus-node_exporter default
-        service prometheus-node_exporter start
+        rc-service prometheus-node_exporter start
     fi
 else
     version=0.14.0

dist/common/scripts/scylla_cpuscaling_setup

@@ -24,7 +24,7 @@ elif is_gentoo_variant; then
         systemctl start cpupower-frequency-set.service
     else
         rc-update add cpupower default
-        service cpupower start
+        rc-service cpupower start
     fi
 else
     yum install -y cpupowerutils || true

dist/common/scripts/scylla_ntp_setup

@@ -44,7 +44,7 @@ elif is_gentoo_variant; then
         systemctl start sntpd.service
     else
         rc-update add ntpd default
-        service ntpd start
+        rc-service ntpd start
     fi
 else
     yum install -y ntp ntpdate || true