[OpenStack] Handle missing services better
[OpenStack] Handle missing services better

Administrators can configure OpenStack tenants and users in such a way that
restricts access to certain services on certain tenants.  The
`OpenstackHandle#detect_service` method handled this by returning `nil` when it
could not connect to the appropriate service.

However, several callers were not handling the `nil` response properly.

Now, at least callers to `OpenstackHandle#service_for_each_accessible_tenant`
will only have their block called when the requested service is actually
available for the tenant.  Otherwise, a warning will be logged, and that
iteration of the `service_for_each_accessible_tenant` will be skipped.

https://bugzilla.redhat.com/show_bug.cgi?id=1245511

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