* Handle failed country code conversion.
* Avoid potential issues with a data-race due to shared slice.
Updates #6028
Change-Id: If7beef2619abd084e1f4109de2d323f834a6090a
placement.AllowedCountry is the old way to specify placement, with the new approach we can use a more generic (dynamic method), which can check full node information instead of just the country code.
The 90% of this patch is just search and replace:
* we need to use NodeFilters instead of placement.AllowedCountry
* which means, we need an initialized PlacementRules available everywhere
* which means we need to configure the placement rules
The remaining 10% is the placement.go, where we introduced a new type of configuration (lightweight expression language) to define any kind of placement without code change.
Change-Id: Ie644b0b1840871b0e6bbcf80c6b50a947503d7df
