Update transit secret key to support auto_rotate_period

[benashz]

The provider originally shipped with support for the
auto_rotate_interval field, this was subsequently changed in the GA
release of Vault to be auto_rotate_period. This fix deprecates the
former and adds the latter.

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Relates OR Closes #0000

Release note for CHANGELOG:

Output from acceptance testing:

$ make testacc TESTARGS='-v -test.run TestTransit*'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -v -test.run TestTransit* -timeout 30m ./...

[...]

ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
=== RUN   TestTransitSecretBackendKey_basic
--- PASS: TestTransitSecretBackendKey_basic (4.94s)
=== RUN   TestTransitSecretBackendKey_rsa4096
--- PASS: TestTransitSecretBackendKey_rsa4096 (8.62s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     14.425s
...