Aggregates metadata cleanup

Adapt review comments on https://review.openstack.org/#/c/112874/
to admin/aggregates/views.py.

Closes-Bug: #1369677

Co-Authored-By: Bartosz Fic <bartosz.fic@intel.com>
Co-Authored-By: Pawel Skowron <pawel.skowron@intel.com>

Change-Id: If75e9e55811fe7cc092080827e69b9ebcd9c9bca
Merge "Aggregates metadata cleanup"