To standarize variable name across roles, this change renames
`horizon_enable_ssl` to `horizon_backend_ssl`.
All other roles use `<service>_backend_ssl` format.
It also better describes what it does. With `horizon_enable_ssl` it's
unclear whether it is about frontend or backend.
Backward compatibility will be kept until 2024.1.
Change-Id: I218d45b7be667732e4204316b8d18fa3e136962b
4686326650