4f993fdd8e
Add 'nova_scheduler_extra_filters' to allow operators to enable
additional scheduler filters without overriding the entire list of
filters as in [0].
This also reduces the burden on ops to maintain the list of overridden
default schedulers because of things like [1].
[0] 3886dbc40d/openstack_deploy/user_variables.yml (L51-L69)
[1] https://review.openstack.org/#/c/596502/
Change-Id: I9ab6bcbef2b496df7f6ecf11a7d8f5f7891aeeca