Use the right backend for TLS tests

The TLS features were introduced to be consummed by the
dogpile.cache.pymemcache backend [1][2], however, the tests
modified by this commit do not instantiate the
dogpile.cache.pymemcache backend but rather the oslo_cache.dict
backend which won't consumme the TLS arguments.

These tests do not really reflects the thing they were made for.

The oslo.cache.dict backend do not expect TLS options [3] whereas
the dogpile.cache.memcached module (and backend) expect TLS
context [4].

This patch propose to switch to the right backend.

[1] https://opendev.org/openstack/oslo.cache/commit/996036acd7b1376f0a378a407697afa28e5b2861
[2] https://github.com/sqlalchemy/dogpile.cache/commit/a2e25bc743a48d91705d944e52aa667ebff10943
[3] https://opendev.org/openstack/oslo.cache/src/branch/master/oslo_cache/backends/dictionary.py
[4] https://github.com/sqlalchemy/dogpile.cache/search?q=tls_context

Change-Id: Ia28ce314044f5790372e6a75dd5d6ae0407bec74