Add option to specify CA for self-signed certificate

This feature request is primarily related to InfluxDB Enterprise.

__Proposal:__
Current setting allow encrypting traffic between InfluxDB nodes with self-signed certificates only with `https-insecure-tls = true`.
It would be nice to add a parameter which allows specifying path to a CA file, so the node's certificate is validated with that CA.

__Current behavior:__
According to the documentation, you need to set `https-insecure-tls` to `true`. With that parameter, self-signed certificates work.

__Desired behavior:__
Introduce a parameter which allows specifying path to CA certificate. For example, `https-ca`.

```
[meta]
[...]
https-enabled = true
https-certificate = "influxdb-meta.crt"
https-private-key = "influxdb-meta.key"
https-ca = "influxdb-ca.crt"
https-insecure-tls = false
```

This parameter needs to be introduced for InfluxDB data and meta nodes' config.

__Alternatives considered:__
CA which issued self-signed certificates can be added to VM's CA bundle, but it's a bit inconvenient.

__Use case:__
Encrypting internal traffic with self-signed certificates is a quite common practice, so allowing to encrypt the traffic in a more secure way would be a good security improvement.

No files or tests are named. Start by tracing HTTPS configuration for InfluxDB data and meta nodes, then determine how certificate validation currently handles https-insecure-tls. Done means both node types accept the proposed CA path and validate self-signed certificates without requiring insecure TLS.

Written by the indexing model from the issue text.