NATS server TLS missing ciphersuite settings when CLI flags used

Summary

(This advisory is canonically <https://advisories.nats.io/CVE/CVE-2021-32026.txt>)

### Problem Description

The NATS server by default uses a restricted set of modern ciphersuites for TLS. This selection can be overridden through configuration. The defaults include just RSA and ECDSA with either AES/GCM with a SHA2 digest or ChaCha20/Poly1305.

The configuration system allows for extensive use of CLI options to override configuration settings. When using these to set a key/cert for TLS, the restricted ciphersuite settings were lost, enabling all ciphersuites supported by Go by default.

None of these additional ciphersuites are broken, so the NATS maintainers have fixed this in public git and the next release is not being hurried, nor is this security advisory embargoed.

### Affected versions

NATS Server: * All versions prior to 2.2.3 * fixed with nats-io/nats-server commit ffccc2e1bd (2021-04-29)

### Impact

If a server administrator chooses to start the nats-server with TLS configuration parameters provided on the command-line, then clients can negotiate TLS ciphersuites which were not expected.

### Workaround

Use a configuration file to set the TLS parameters instead of command-line options.

### Solution

Upgrade the NATS server.

### Credits

This issue was identified and reported by SimCorp.