add --tls flag by default for redis client & add unit test

[markdboyd]

Changes proposed in this pull request:

• add --tls flag by default for redis client
• add unit test

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

The TLS flag is necessary for connecting to our Redis instances because they are all configured to use TLS, as documented here: https://cloud.gov/docs/services/aws-elasticache/#connecting-to-your-elasticache-service-locally

[cweibel]

Adds tls flag and test functionality