MT#12513 Check $RTP_INTERFACES_CONFIG availability for sync-db

Otherwise users notice error during upgrade mr3.7->mr3.8:
> ngcpcfg commit 'snapshot during /usr/share/ngcp-upgrade-mr3.8/upgrade-2.8-to-mr3.8 - Mon Apr 13 10:29:24 CEST 2015'
> YAML Error: Couldn't open /etc/ngcp-rtpengine-daemon/interfaces.yml for input:\nNo such file or directory
>   Code: YAML_LOAD_ERR_FILE_INPUT
> at /usr/share/perl5/YAML.pm line 71
> OK
> Synchronizing data from /etc/ngcp-config/constants.yml

Change-Id: I2e4b8780d2ada64854186da93b03b8817eac99bf
(cherry picked from commit 92536298f7)

scripts/commit

@@ -18,7 +18,11 @@ HELPER="${HELPER:-/usr/share/ngcp-ngcpcfg/helper/}"
 cd "$NGCPCTL_MAIN"
 
 if [ -z "${NO_DB_SYNC:-}" ] ; then
+  if [ -f "${RTP_INTERFACES_CONFIG}" ] ; then
     "${HELPER}/sync-db" "$NGCPCTL_CONFIG" "$CONSTANTS_CONFIG" "$RTP_INTERFACES_CONFIG" || true
+  else
+    "${HELPER}/sync-db" "$NGCPCTL_CONFIG" "$CONSTANTS_CONFIG" || true
+  fi
 else
     log_debug "no-db-sync: skipping 'sync-db'"
 fi