Document how to start a new agent after 6.3.0 mtls changes

[aplanas]

After this commit: keylime/keylime@70a2f8e that is part of keylime 6.3.0, an agent cannot be started until the CA certificate (that is usually living in the register / verifier node) is copied into the agent node.

We should document this step, together with strategies that allow easy new agent deployments.

[aplanas]

cc: @THS-on

[aplanas]

I updated the doc in the openSUSE MicroOS portal: https://en.opensuse.org/Portal:MicroOS/RemoteAttestation#Keylime_agent

[mpeters]

I thought the upgrade would continue to work with previous agents on non-mtls so there would exist an upgrade path. Maybe there's a bug that needs to be fixed @THS-on ?

[THS-on]

@mpeters the old agents (<6.3) still work, but we haven't updated the documentation for the 6.3 agents which now require a the CA for the mTLS connections.