Add desired host phase 2 contents to OmicronSledConfig (PR 1/4)

[jgallagher]

HostPhase2DesiredContents is analogous to OmicronZoneImageSource, but for OS images: either keep the current contents of the boot disk or set it to a specific artifact from the TUF repo depot. "Keep the current contents" should show up in three cases, just like OmicronZoneImageSource::InstallDataset:

1. It's the default value for deserializing, so we can load old configs that didn't have this value
2. RSS uses it (no TUF repo depot involved at this point)
3. The planner will use this variant as a part of removing a mupdate override (this work is still in PR itself: [29/n] blueprint planner logic for mupdate overrides #8456 (comment))

[jgallagher]

This and its friend below are both addressed in #8539

[andrewjstone]

TIL

[sunshowers]

thanks!