MRC: made Vivaldi-based replica selection policy accessible, improved...

MRC: made Vivaldi-based replica selection policy accessible, improved robustness when trying to use non-existing policies

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@1395 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534