[snmp] Configure snmp docker hostname from config DB (#2773)

* [snmp] Configure snmp docker hostname from config DB
* Fixed reviewer comments

dockers/docker-snmp-sv2/start.sh

@@ -9,6 +9,17 @@ sonic-cfggen -d -y /etc/sonic/snmp.yml -t /usr/share/sonic/templates/snmpd.conf.
 mkdir -p /var/sonic
 echo "# Config files managed by sonic-config-engine" > /var/sonic/config_status
 
+CURRENT_HOSTNAME=`hostname`
+HOSTNAME=`sonic-cfggen -d -v DEVICE_METADATA[\'localhost\'][\'hostname\']`
+
+if [ "$?" == "0" ] && [ "$HOSTNAME" != "" ]; then
+    echo $HOSTNAME > /etc/hostname
+    hostname -F /etc/hostname
+
+    sed -i "/\s$CURRENT_HOSTNAME$/d" /etc/hosts
+    echo "127.0.0.1 $HOSTNAME" >> /etc/hosts
+fi
+
 rm -f /var/run/rsyslogd.pid
 
 supervisorctl start rsyslogd