Rhbz1139950 filename mapping

~~this depends on https://github.com/zanata/zanata-api/pull/18~~
How about we name this `getRelativeTransFilePathForSourceDoc`
I know it's a bit on the long side, but it clearly represents what is happening here, which is to get the expected path of a translation file in a given locale for a source document.
(Feel free to find a shorter name that conveys the same)