Sleds that joined trust quorum but failed to initialize cannot be added again

[wfchandler]

This scenario seems like a consequence of #4494.

Should an operator call rack_membership_add_sled, the two broad steps are:

1. Add the new sled to TQ
2. Send StartSledAgentRequest to the sled

Should the sled successfully join TQ, but fail in StartSledAgentRequest before it has written its ledger, subsequent calls to rack_membership_add_sled will fail with The following sleds are already members of the trust quorum:....

The operator has no way to remove the sled from TQ, so manual intervention by support is required to resolve the situation.

[jgallagher]

I think #11064 (will ship in R23) should have fixed the general problem here - Nexus should now periodically retry adding sleds that have been added to TQ but are not yet initialized.