luci-app-ddns: make service-specific 'cacert' optional

[bam80]

Seems like it should be optional as we have global 'cacert' setting above

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[systemcrash]

Seems OK - @Ansuel ?

[systemcrash]

Merged. Thanks @bam80

[bam80]

@systemcrash You are welcome but I didn't check this patch, hope you did it before the merge