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).
removed openstack common play as its no longer used
Merge pull request #225 from cloudnull/removed-openstack-common

Removed openstack common play as its no longer used
Merge pull request #223 from mattt416/trello_35

Update horizon's local_settings.py template for Juno