This commit fixes the nats TLS tests by generating new certificates
(root CA, server and client) - each valid for 10y. The new certificates
don't have a common name (deprecated by X.509) but SANs instead.
Since Go 1.15 the Go `crypto/x509` package rejects certificates that
only have a common name and no SAN. See: https://golang.org/doc/go1.15#commonname
- added ability to specify CA for self-signed certificates
- added option to authenticate using client certificates
- added unit tests for nats connections