Detect failure to prepare installation

If the status for the hosts in assisted-installer changes from preparing-for-installation to ready, that means that it failed to generate the ignition configs needed to install, and installation will not proceed. When we see this we should report a failure immediately from agent wait-for bootstrap-complete. Currently we just time out some time after reporting this log message:

level=info msg=Host master-2.ostest.test.metalkube.org: updated status from preparing-for-installation to known (Host is ready to be installed) 

To catch the case where the user runs the command after this failure has already happened, perhaps we should institute a relatively short timeout for installation to begin after all of the hosts are in the known state.