Create cache dir hierarchy when using alien_cache.
Merge pull request #311 from btovar/alien_cache_create_hierarchy

Create cache dir hierarchy when using alien_cache.

This pull request is to ensure the creation of the cache directory hierarchy when using the alien_cache option. If alien_cache is being used, the hierarchy is created with 0770 permissions, and the EEXIST error is ignored, in case two processes are racing to create the hierarchy.