Concatenate ini files in inventory directory

This pull request follows a discussion on the project mailing list : https://groups.google.com/forum/#!topic/ansible-project/d51VnOdLa_o

When parsing an inventory in a directory, all the "ini" files are now concatenated in a temp file before being parsed all in one shot.

This allows to declare objects in a file and reference them in the next file, to efficiently split an "ini" inventory in many files. For instance : `prod/01_hosts`, `prod/02_role_groups`, `prod/03_client_groups` and `prod/04_realm_groups`