Update horizon's local_settings.py template for Juno

This change brings in some of the differences in Juno's
local_settings.py file and removes FLAVOR_EXTRA_KEYS since that is now
deprecated.

Some opinion has been applied to supported_provider_types by removing
the ability to create local and gre networks via horizon.  This can be
adjusted if anyone believes these should be added (or others removed).
Merge pull request #223 from mattt416/trello_35

Update horizon's local_settings.py template for Juno