This variable is referenced by horizon_listen_port, which becomes
undefined outside of the horizon role. One symptom of this is that
the hostvars variable becomes undefined when referenced for debugging
purposes.
This issue was introduced by Ibb5ad1a5d1bbc74bcb62610d77852d8124c4a323,
which has been backported to Victoria.
This change fixes the issue by moving horizon_enable_tls_backend to
group_vars.
TrivialFix
Change-Id: I1fc4e2a24fe096a49434d7e16851e63efd25d74c
4c6d777868