Find out how bootstrap node is chosen, make it be node0 and delay the reboot

Currently assisted service chooses one of the nodes that reach out to it to be the bootstrap node. We need to understand the choice mechanism and to make it reliably choose the node that we want node0 to be.

The bootstrap node already waits for the other nodes before rebooting, we need to make sure that this wait is sufficient for assisted-service as well. Prevent the assisted-service from rebooting the node it is running on until the following conditions are true:

1. Installation is complete on all other hosts
2. The cluster control plane is up and accessible

We can try with having it reboot into bootstrap while making sure that assisted-service runs after reboot but ideally we'd want to have the node start bootstrapping without needing the reboot (As per customer/PM demands to minimize reboots).

In the context of METAL-10 there was a proposal to add a file that the agent would check for, such that the presence of this file would inhibit a reboot. We could possibly use the same mechanism here to avoid the need for large-scale changes to how assisted-service itself works (assisted-service would still need to delete the file at the appropriate time, but that is a less-invasive change). However, there are timeouts that have to be considered, so changes to the state machine may be required.

Note that we do want to continue to install to disk on the assisted-service host in parallel with the others, since this is on the critical path slowing down all deployments. Only the reboot should be delayed.

Single-node deployments are an exception to this.