Align all env variables for cluster management in all languages

[vic-tsang]

• renamed all env variables to use the standard ones

Single Cluster

• CLUSTER_REGION
• CLUSTER_ID

Multi Clusters

• CLUSTER_1_REGION
• CLUSTER_2_REGION
• WITNESS_REGION
• CLUSTER_1_ID
• CLUSTER_2_ID

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[danielfrankcom]

Changes here mostly look good.

I did a quick search in the branch and found the following which I think were missed:

• go/cluster_management/cmd/update_cluster/update_cluster_integ_test.go

			region:     os.Getenv("REGION"),
			identifier: os.Getenv("CLUSTER_ID"),

• go/cluster_management/cmd/get_cluster/get_cluster_integ_test.go

			region:     os.Getenv("REGION"),
			identifier: os.Getenv("CLUSTER_ID"),

For whatever reason it seems these are also setting environment variables but any change to that is likely out of scope. We should use consistent names imo. For extra weirdness points, the create_multi_region_integ_test.go file and the single-region equivalent don't use environment variables at all. Even the ones I mentioned above inconsistently reference the region, sometimes with an env var and sometimes hardcoded.