join token missing check

[kfox1111]

The join token missing check currently happens at node attestation time. when combined with retry_bootstrap=true, it can log out multiple times
WARN[0004] Failed to retrieve attestation result error="rpc error: code = InvalidArgument desc = nodeattestor(join_token): join token was not provided" retry_interval=7.792022514s

It may be better to push up the check more towards startup.

If not possible to move ujp, its a fatal error and should probably exit asap rather then continue on indefinitely.