Scheduler: return a namedtuple from _get_group_details

Commit 459ca56de2366aea53efc9ad3295fdf4ddcd452c added the utility
method _get_group_details. This patch returns a namedtuple that
contains the hosts and policies for the instance group. This makes
the code easier to read and more robust.

TrivialFix

Change-Id: I76208092b06408022f48b1d965b982af86c78161
Merge "Scheduler: return a namedtuple from _get_group_details"