[for-0.12] luci-app-ddns: fix OpenWrt Ticket #18018 in BB release

fix for OpenWrt Ticket #18018 in BB 14.07 release.
Without fix the application writes a config without event interface
which causes ddns-scripts to NEVER start.

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>
Merge pull request #224 from chris5560/luci-0.12

[for-0.12] luci-app-ddns: fix OpenWrt Ticket #18018 in BB release