Improve TLS configuration for certs

[massenz]

Currently the location of the TLS cryptographic material (certificate and keys) is in a fixed location (/etc/statemachine/certs) and we only have the -insecure flag to disable TLS.

We should have a more flexible setup - continuing to enable TLS by default and keeping the default location, however we should also allow:

• keep the -insecure flag to disable TLS;
• add a -tlsCertFile for the X509 cert;
• add a -tlsKeyFile for the certificate private key;
• add a -tlsCAFile for an (optional) Certificate Authority root file (self-signed certs)

If not configured, the defaults will be:

/etc/statemachine/certs/server.pem for the certificate
/etc/statemachine/certs/server-key.pem for the private key
No CA file (roots as installed by the host OS)