Azure Creds: Honors the documented default behavior (again)

[TheLonelyGhost]

If no environment is specified, will show up as an empty string and (currently) throw an error that "" is an unknown cloud provider. After this change it allows the environment attribute on the datasource to be optional again.

Ideally this would fetch the value from Vault, but for most cases we may want the default to be Azure Public Cloud, which is the default on the Vault backend if nothing is specified.

Description

Documented behavior of the azure credentials data source is that environment is optional. In reality, not providing it (only in v4.0.0) results in an error message saying "" is an unknown cloud provider.

Closes #2189

Checklist

• Added CHANGELOG entry (only for user-facing changes)
• Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccDataSourceAzureAccessCredentials'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -run=TestAccDataSourceAzureAccessCredentials -timeout 30m ./...
?       github.com/hashicorp/terraform-provider-vault   [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/coverage      [no test files]
?       github.com/hashicorp/terraform-provider-vault/cmd/generate      [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/consts   [no test files]
?       github.com/hashicorp/terraform-provider-vault/helper    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/codegen   0.319s [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/group   [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/identity/mfa     [no test files]
?       github.com/hashicorp/terraform-provider-vault/internal/pki      [no test files]
ok      github.com/hashicorp/terraform-provider-vault/internal/identity/entity  (cached) [no tests to run]
?       github.com/hashicorp/terraform-provider-vault/internal/sync     [no test files]
?       github.com/hashicorp/terraform-provider-vault/schema    [no test files]
?       github.com/hashicorp/terraform-provider-vault/util/mountutil    [no test files]
ok      github.com/hashicorp/terraform-provider-vault/internal/provider (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/testutil  (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
ok      github.com/hashicorp/terraform-provider-vault/vault     0.654s

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[hashicorp-cla]

All committers have signed the CLA.

[fairclothjm]

Thanks @TheLonelyGhost !