agent: fill/generate real TrustAnchorCerts in last report progress

[glimchb]

see

sztp/sztp-agent/pkg/secureagent/daemon.go

Lines 103 to 105 in f969315

	// TODO: use/generate real TA cert here
	encodedKey := base64.StdEncoding.EncodeToString([]byte("mysshpass"))
	p.IetfSztpBootstrapServerInput.TrustAnchorCerts.TrustAnchorCert = []string{encodedKey}

[bhoopesh369]

@glimchb can you assign me to this issue?