feat(canopsis-reconfigure): Use a convenient default value for -mongoConf, as was done for -conf

@mmourcia This unbreaks canopsis-reconfigure in package installations (when it's called by canoctl deploy). And maybe in the canopsis-reconfigure Docker image too, but I didn't test that one yet.

Let's just use a default value value for -mongoConf, as was already done for the -conf flag.