changed get_group to run in O(n*log(n)) instead of O(n^2) by using _grou...

...ps_cache_hash

TODO: check if cache needs to be invalidated in more places (someone who knows the code should check for that...)

ansible startup time goes down by 12 seconds with 12k groups in the inventory. Searching in a list is just slow.

See also: https://groups.google.com/forum/#!topic/ansible-devel/YB-6SRvS5MU