Agent and server log output could be improved during attestation

[ajessup]

When reading the logs of the spire-agent and spire-server during node attestation, it would be useful to be able to see the node attestation strategy employed by the agent.

For example, in the agent:

INFO[0000] Preparing to attest against %s127.0.0.1:8081
INFO[0000] Generating a CSR for %sspiffe://example.org/spiffe/node-id/NOT-A-SECRET
INFO[0000] Attestation complete

could be:

INFO[0000] Preparing to attest this node using strategy `join-token`
INFO[0000] Generating a CSR for spiffe://example.org/spiffe/node-id/NOT-A-SECRET
INFO[0000] Node attestation complete

And on the server, when a node is attested, instead of nothing in the logs, we could have something more informative such as:

INFO[0000] Received node attestation request from spiffe://example.org/spiffe/node-id/NOT-A-SECRET using strategy `join-token`... completed successfully. SVID issued with TTL=300s.