propolis should allow for control over boot order

[pfmooney]

Placeholder issue relating to RFD 470 and oxidecomputer/customer-support#84

[pfmooney]

This should be easier to implement with once #705 is merged.

[iximeow]

we've got some limited control here as of #756, where you can now specify a list of boot disks to propolis-server. Nexus then only ever records and provides a single boot disk. for the _why_s on the relatively limited implementation, i've added some more discussion to RFD 470.