Add local to audit resource

[jasonodonnell]

This adds the local parameter to the audit backend resource to configure the replication setting of the audit backend. This resource was using a deprecated SDK method so I can changed it to the newer one (EnableAuditWithOptions).

* `resource/vault_audit `: added support for local mount to configure replicating the audit backend

Output from acceptance testing:

$ TESTARGS="--run TestResourceAudit" make testacc

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v --run TestResourceAudit -timeout 120m
?   	github.com/terraform-providers/terraform-provider-vault	[no test files]
?   	github.com/terraform-providers/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/terraform-providers/terraform-provider-vault/cmd/generate	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/codegen	(cached) [no tests to run]
?   	github.com/terraform-providers/terraform-provider-vault/generated	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/datasources/transform/decode	1.309s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/datasources/transform/encode	0.721s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/alphabet	1.047s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/role	2.817s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/template	2.006s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/transformation	0.997s [no tests to run]
?   	github.com/terraform-providers/terraform-provider-vault/schema	[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   TestResourceAudit
--- PASS: TestResourceAudit (0.15s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	2.671s

[tomhjp]

LGTM