Vary twemproxy config location based on getRealmSpecificFilename() (take 2)

Move the actual execution into script and not pre-start script since
variables can not make it from one script to another. This is not really
DRY but it works in a rather cleaner way than passing variables through
temp files.

Bug: 62836
Change-Id: Ic681f577871e707b16d51afb46d2e07995827724