Show sds config name in /certs when Secret Discovery Service is used

*Title*: Show sds config name in /certs when Secret Discovery Service is used

*Description*:
the `/cert` admin path shows the certificate name used in Envoy and the this is very useful for debugging as it refers to the certificate used in the listener/cluster tls config.

The following is the `/certs` output when SDS is enabled:
```yaml
...
"ca_cert": [
{
"path": "\u003cinline\u003e",
"serial_number": "116db900f6bb4bd4a3fcd8fa57a1b141",
"subject_alt_names": [],
"days_until_expiration": "36196",
"valid_from": "2018-04-25T21:11:33Z",
"expiration_time": "1982-03-19T15:43:17Z"
}
],
"cert_chain": [
{
"path": "\u003cinline\u003e",
"serial_number": "0287be20c0bc92ce9d87b5f5f396f2ef1d7670",
"subject_alt_names": [
{
"uri": "my-service-1"
}
],
"days_until_expiration": "0",
"valid_from": "2019-03-18T22:43:59Z",
"expiration_time": "2019-03-19T22:48:59Z"
}
]
},
{
"ca_cert": [
{
"path": "\u003cinline\u003e",
"serial_number": "116db900f6bb4bd4a3fcd8fa57a1b141",
"subject_alt_names": [],
"days_until_expiration": "36196",
"valid_from": "2018-04-25T21:11:33Z",
"expiration_time": "1982-03-19T15:43:17Z"
}
],
"cert_chain": [
{
"path": "\u003cinline\u003e",
"serial_number": "0287be20c0bc92ce9d87b5f5f396f2ef1d7670",
"subject_alt_names": [
{
"uri": "my-service-2"
}
],
"days_until_expiration": "0",
"valid_from": "2019-03-18T22:43:59Z",
"expiration_time": "2019-03-19T22:48:59Z"
}
]
},
...
```
When SDS is enabled, only `` is showed for all certs in the dump which makes the `/certs` page kind of useless as you cannot link it back to the sds config name in listener/cluster tls config.

*Expected Behavior*:
The sds config name should be used in the `/certs` output.
https://github.com/envoyproxy/envoy/blob/34532dd6d636e5b0f6cbc42e84c554a78eb34b1d/api/envoy/api/v2/auth/cert.proto#L367

@JimmyCYJ @lizan @htuch thoughts?