Jwt auth import

[catsby]

Fixes #295 . This resource uses path for the ID, but because(?) it's a ForceNew attribute it was never read from the API. Attempting to import uses an ID input, but the path that was used for the READ operation was empty so no data was found. This will set the path to the Id() value if path is not found, so the READ should succeed.

In order to import without changes needed, this PR also sets tune and type information.

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Relates OR Closes #295

Release note for CHANGELOG:

- Update jwt_auth_backed to enable importing existing backends. 
- Update jwt_auth_backed to read `type` and `tune` information

Output from acceptance testing:

[terraform-provider-vault][jwt_auth_import](4)$ make testacc TESTARGS='-run=TestAccJWT'                                                                                         ✭
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccJWT -timeout 120m
?       github.com/terraform-providers/terraform-provider-vault [no test files]
?       github.com/terraform-providers/terraform-provider-vault/cmd/coverage    [no test files]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/util    (cached) [no tests to run]
=== RUN   TestAccJWTAuthBackendRole_import
--- PASS: TestAccJWTAuthBackendRole_import (0.14s)
=== RUN   TestAccJWTAuthBackendRole_basic
--- PASS: TestAccJWTAuthBackendRole_basic (0.11s)
=== RUN   TestAccJWTAuthBackendRole_update
--- PASS: TestAccJWTAuthBackendRole_update (0.18s)
=== RUN   TestAccJWTAuthBackendRole_full
--- PASS: TestAccJWTAuthBackendRole_full (0.11s)
=== RUN   TestAccJWTAuthBackendRoleOIDC_full
--- PASS: TestAccJWTAuthBackendRoleOIDC_full (0.83s)
=== RUN   TestAccJWTAuthBackendRole_fullUpdate
--- PASS: TestAccJWTAuthBackendRole_fullUpdate (0.19s)
=== RUN   TestAccJWTAuthBackendRole_fullDeprecated
--- PASS: TestAccJWTAuthBackendRole_fullDeprecated (0.19s)
=== RUN   TestAccJWTAuthBackend
--- PASS: TestAccJWTAuthBackend (0.96s)
=== RUN   TestAccJWTAuthBackend_OIDC
--- PASS: TestAccJWTAuthBackend_OIDC (0.22s)
=== RUN   TestAccJWTAuthBackend_negative
--- PASS: TestAccJWTAuthBackend_negative (0.03s)
=== RUN   TestAccJWTAuthBackend_missingMandatory
--- PASS: TestAccJWTAuthBackend_missingMandatory (0.55s)
PASS
ok      github.com/terraform-providers/terraform-provider-vault/vault   3.773s


...

[tvoran]

Just some wondering about the log formatting, non-blocking.