To standarize variable name across roles, this change renames
`keystone_ssl` to `keystone_backend_ssl`.
All other roles use `<service>_backend_ssl` format.
It also better describes what it does. With `keystone_ssl` it's unclear
whether it is about frontend or backend.
Backward compatibility will not be implemented because securing haproxy
traffic to its backends with TLS is currently not supported by OSA so
it is hard to leverage `keystone_ssl` variable anyway.
Change-Id: Ibf8607a4cf62ab518a09d64b1054ff7fbc580000
6661a9dab7