Recommend putting AZ in locality flag for cloud deployments

[bdarnell]

Benjamin Darnell (bdarnell) commented:

If the AZ is not reflected in the --locality flag, then ranges may not be properly diversified across locality zones and will be at an increased risk of outage during an availability zone failure. We should update our production recommendations/best practices pages for cloud deployments to incorporate this:

• In most cases, for deployments to the big three clouds, the locality flag should include both the region and AZ, and look like --locality=region=us-east-1,az=us-east-1c. The region portion is optional for single-region clusters, but it doesn't hurt to always include it. Users may add additional levels to this if they wish.
• Zone configs should generally reference regions, not AZs. The user will not need to refer to the AZs anywhere outside the --locality flag, but it will be used by the replica placement system to ensure proper diversity.
• Different regions can have different numbers of availability zones (Most have three, but consult your cloud's documentation. For aws: https://aws.amazon.com/about-aws/global-infrastructure/). Prefer regions with at least three AZs whenever possible.

Jira Issue: DOC-264

[mwang1026]

cc @piyush-singh seems server-y?

[jseldess]

Definitely. This relates to #10311, actually, which @taroface is taking under Deploy and Ops. @taroface, feel free to consolidate these.