This adds support for configuring certificate/key files used for TLS
connection with Redis using the existing tls_* options.
example)
[cache]
backend=dogpile.cache.backend.redis
backend_arguments=url:rediss://:a_big_secret@localhost:2679
tls_enabled=True
tls_cafile = /etc/pki/tls/certs/ca-bundle.crt
Change-Id: I2ab38b8c88274cb4908791eea8212a79e3d524a2