add support for pki auth

[realmgic]

No description provided.

[spkesan]

What do you think about having validations in conjunction with TLS configurations when PKI auth mode is used. I'll let @mphanias chip in.

[spkesan]

Suggested change

	if authMode == "external" {
	clientPolicy.AuthMode = aero.AuthModeExternal
	} else if authMode == "pki" {
	clientPolicy.AuthMode = aero.AuthModePKI
	switch authMode {
	case "internal":
	clientPolicy.AuthMode = aero.AuthModeInternal
	case "external":
	clientPolicy.AuthMode = aero.AuthModeExternal
	case "pki":
	clientPolicy.AuthMode = aero.AuthModePKI
	default:
	log.Fatalln("Invalid auth mode: only `internal`, `external`, `pki` values are accepted.")
	}

and remove line 85-86

[realmgic]

Fixed. I also added the default value of "" as an option.

[spkesan]

Thanks @realmgic .

Two things,

1. Can you create this PR against dev branch ?
2. And, we might want to add a validation such that when authMode is pki, clientPolicy.TlsConfig.Certificates (which contains the certiticates that the client presents to the server) must not be empty. What do you think?

[spkesan]

Otherwise, LGTM. I'll let @mphanias take a look.