Better sorting of flavors in the "Launch Instance" view

By default, returns the available flavors, sorted by RAM usage
(ascending).  Override these behaviours with a CREATE_INSTANCE_FLAVOR_SORT
dict in local_settings.py.

DocImpact: a new django setting, CREATE_INSTANCE_FLAVOR_SORT.

Change-Id: I9f466ff306739369a85f7176da27932958fc8867
Fixes: Bug 1196690