Flavors metadata cleanup

Fix review comments on https://review.openstack.org/#/c/112874/
in admin/flavors/views.py regarding exception handling. Also
adding unit test to test negative scenario after changes.

Closes-Bug: 1369678
Co-Authored-By: Bartosz Fic <bartosz.fic@intel.com>
Co-Authored-By: Michal Dulko <michal.dulko@intel.com>
Change-Id: I5ea1216a22560cc0e8a5ba57be548b54674db689
Merge "Flavors metadata cleanup"