tests/resource/aws_ec2_client_vpn_endpoint: Use internal implementation for TLS key/certificate
tests/resource/aws_ec2_client_vpn_endpoint: Use internal implementation for TLS key/certificate

Reference: https://github.com/terraform-providers/terraform-provider-aws/issues/10023

Output from acceptance testing:

```
--- PASS: TestAccAwsEc2ClientVpnEndpoint_disappears (18.01s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_basic (17.22s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_splitTunnel (29.09s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_withDNSServers (29.77s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_withLogGroup (36.80s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_tags (39.35s)
--- PASS: TestAccAwsEc2ClientVpnEndpoint_msAD (1761.91s)
```

kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws