Redis Sentinel Separation

Closes gitlab-org/quality/performance#156 (closed) gitlab-org/quality/performance#164 (closed)

MR allows for more flexibility for Redis Sentinel as per the following requirements:

• Redis can now be deployed as a single combined cluster or as multiple clusters based on Redis Class
• Redis Sentinel cluster can now be deployed onto it's own nodes (for redis single or multiple clusters) or onto the consul nodes (only for redis single cluster).
• Has other small improvements such as being able to override the gitlab package version as required. Also updates the Terraform version to 0.12.18 and locks it (switch to running apt-get directly instead of Ansible's apt module as the latter is too limited to handle downgrades correctly).