Flush DB first when loading from minigraph (sonic-net#114)

* Flush DB first when loading from minigraph and add a config reload command to flush db and reload from json.

config/main.py

@@ -118,6 +118,20 @@ def load(filename):
     command = "{} -j {} --write-to-db".format(SONIC_CFGGEN_PATH, filename)
     run_command(command, display_cmd=True)
 
+@cli.command()
+@click.option('-y', '--yes', is_flag=True, callback=_abort_if_false,
+                expose_value=False, prompt='Clear current and reload all config?')
+@click.argument('filename', default='/etc/sonic/config_db.json', type=click.Path(exists=True))
+def reload(filename):
+    """Clear current configuration and import a previous saved config DB dump file."""
+    config_db = ConfigDBConnector()
+    config_db.connect()
+    client = config_db.redis_clients[config_db.CONFIG_DB]
+    client.flushdb()
+    command = "{} -j {} --write-to-db".format(SONIC_CFGGEN_PATH, filename)
+    run_command(command, display_cmd=True)
+    client.set(config_db.INIT_INDICATOR, True)
+
 @cli.command()
 @click.option('-y', '--yes', is_flag=True, callback=_abort_if_false,
                 expose_value=False, prompt='Reload mgmt config?')
@@ -135,14 +149,20 @@ def load_mgmt_config(filename):
     run_command(command, display_cmd=True)
     command = "[ -f /var/run/dhclient.eth0.pid ] && kill `cat /var/run/dhclient.eth0.pid` && rm -f /var/run/dhclient.eth0.pid"
     run_command(command, display_cmd=True)
+    print "Please note loaded setting will be lost after system reboot. To preserve setting, run `config save`."
 
 @cli.command()
 @click.option('-y', '--yes', is_flag=True, callback=_abort_if_false,
                 expose_value=False, prompt='Reload config from minigraph?')
 def load_minigraph():
     """Reconfigure based on minigraph."""
+    config_db = ConfigDBConnector()
+    config_db.connect()
+    client = config_db.redis_clients[config_db.CONFIG_DB]
+    client.flushdb()
     command = "{} -m --write-to-db".format(SONIC_CFGGEN_PATH)
     run_command(command, display_cmd=True)
+    client.set(config_db.INIT_INDICATOR, True)
     command = "{} -m -v \"DEVICE_METADATA['localhost']['hostname']\"".format(SONIC_CFGGEN_PATH)
     p = subprocess.Popen(command, shell=True, stdout=subprocess.PIPE)
     p.wait()
@@ -159,7 +179,7 @@ def load_minigraph():
     run_command("service lldp restart", display_cmd=True)
     run_command("service snmp restart", display_cmd=True)
     run_command("service dhcp_relay restart", display_cmd=True)
-
+    print "Please note setting loaded from minigraph will be lost after system reboot. To preserve setting, run `config save`."
 #
 # 'bgp' group
 #