[semver:minor] make GOOGLE_COMPUTE_{REGION,ZONE} optional

[wyardley]

Checklist

• [x] All new jobs, commands, executors, parameters have descriptions
• [x] Examples have been added for any significant new features
• [x] README has been updated, if necessary

Motivation, issues

Fixes #38

The changes in #23 make the initialize command fail if one of GOOGLE_COMPUTE_ZONE or GOOGLE_COMPUTE_REGION is not set. While having one of these set implicitly may be useful for some people, it is not required for all commands, and can also usually be specified via --zone, --region, or --location explicitly as well. This also updates docs to make some parameter docs clearer.

Description

Make initialize succeed even if neither of GOOGLE_COMPUTE_ZONE or GOOGLE_COMPUTE_REGION are set. This will also enable both to be set if both of the env vars exist.

Clarify docs:

• Clarify when it's the env var name vs. the value to use
• Clarify wording so it's clear that these are the default values for the CLI to use

[wyardley]

I think this should be patch, but happy to adjust if you think it should be minor

[gmemstr]

I'm marking this as a minor due to the behaviour changes, so fewer people relying on this behaviour should be impacted (if anyone is, would be a bit odd if they were relying on this failstate).