fix: support importing gcp_auth_backend

[benashz]

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Closes #795
Relates to #913

Release note for CHANGELOG:

resource/gcp_auth_backend`: Support importing preexisting resource 

Output from acceptance testing:

$ make dev testacc TESTARGS='-run \^TestGCPAuthBackend$$'
==> Checking that code complies with gofmt requirements...
go build -o terraform-provider-vault
mv terraform-provider-vault ~/.terraform.d/plugins/
TF_ACC=1 go test $(go list ./...) -v -run \^TestGCPAuthBackend$ -timeout 120m

[...]

=== RUN   TestGCPAuthBackend
=== RUN   TestGCPAuthBackend/basic
=== RUN   TestGCPAuthBackend/import
--- PASS: TestGCPAuthBackend (0.31s)
    --- PASS: TestGCPAuthBackend/basic (0.15s)
    --- PASS: TestGCPAuthBackend/import (0.16s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     (cached)

[tvoran]

Seems to work for me!