Add vault_ssh_secret_backend_role resource

[martinssipenko]

This adds a new vault_ssh_secret_backend_role resource that can be used to manage SSH secret backend roles using Terraform. This does not include all the fields that are supported by resource, but should be a good start as I think having something is better than nothing.

I've ran acceptance tests I've written using a local Vault server in dev mode, output below:

make testacc TESTARGS='-run=TestAccSSHSecretBackendRole_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccSSHSecretBackendRole_basic -timeout 120m
?   	github.com/terraform-providers/terraform-provider-vault	[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   TestAccSSHSecretBackendRole_basic
--- PASS: TestAccSSHSecretBackendRole_basic (1.07s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	(cached)

make testacc TESTARGS='-run=TestAccSSHSecretBackendRole_import'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccSSHSecretBackendRole_import -timeout 120m
?   	github.com/terraform-providers/terraform-provider-vault	[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   TestAccSSHSecretBackendRole_import
--- PASS: TestAccSSHSecretBackendRole_import (1.37s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	(cached)

Fixes #246

[tyrannosaurus-becks]

@martinssipenko fantastic work. Thank you!