Fixed TypeError in database launch error handle

openstack_dashboard.dashboards.project.databases.workflows.create_instance
in populate_flavor_choices

TypeError: 'NoneType' object is not iterable

If there's an exception in
openstack_dashboard/dashboards/project/databases/workflows/create_instance.py
SetInstanceDetailsAction.flavors, the follow-on method will have a
TypeError.

[code]
    @memoized.memoized_method
    def flavors(self, request):
        try:
            return api.trove.flavor_list(request)
        except Exception:
            LOG.exception("Exception while obtaining flavors list")
            self._flavors = []

    def populate_flavor_choices(self, request, context):
        flavor_list = [(f.id, "%s" % f.name) for f in self.flavors(request)]
        return sorted(flavor_list)
[code]

if self.flavors has an error, it will eventually return a None.

There's 2 options here:
A) return an empty list
B) redirect to another page

I have to believe that returning an empty list will be more confusing
than a redirect, so I'd like to handle this with exceptions.handle.

Change-Id: I9aff36875c26958ab8ef1f99828ed6113216c3c3
Closes-Bug: 1306815
Merge "Fixed TypeError in database launch error handle"