Fix LeastUsedDistributor and ensure random distribution for multiple non-fs directories

If we cannot determine available space the fallback scenario is now to use random distribution instead of always using the last directory.

Fixes #2820
Added sort mode to geo distance sorting. Closes #1846
Merge branch 'master' of git://github.com/elasticsearch/elasticsearch
Merge branch 'master' of https://github.com/msimons/elasticsearch

# By Igor Motov (1) and Martijn van Groningen (1)
# Via Marco Simons (1) and Martijn van Groningen (1)
* 'master' of https://github.com/msimons/elasticsearch:
  Added sort mode to geo distance sorting. Closes #1846
  Fix LeastUsedDistributor and ensure random distribution for multiple non-fs directories