BAC-854: use $wgImagesDomainSharding in wfReplaceImageServer

We need to a way to change a pattern used for sharding images domain for wikis with Swift storage enabled and disabled.

@Wikia/platform-team @moliwikia 

Merge together with https://github.com/Wikia/config/pull/225 (sets ``$wgImagesDomainSharding``)