Document guidelines w.r.t. where rpool filesystem creation should occur

[prakashsurya]

Following some discussion about delphix/delphix-platform#31, we need to better document where rpool dataset creation should occur, since we can create these datasets in multiple places.

I propose the following approach and/or guidelines:

1. Only create the minimal set of datasets required to boot "correctly" via the appliance-build script that's used to create the initial VM images.. currently, this means the following datasets:

   • rpool/ROOT/delphix.XXXX/root
   • rpool/ROOT/delphix.XXXX/data
   • rpool/ROOT/delphix.XXXX/home
   • rpool/grub

2. Similarly, only create the minimal set of datasets required for "not-in-place" upgrades via the upgrade scripts.. currently, this means:

   • rpool/ROOT/delphix.YYYY/root
   • rpool/ROOT/delphix.YYYY/data
   • rpool/ROOT/delphix.YYYY/home

3. All other datasets should be generated by the delphix-platform service.

In (1), we need to create rpool/grub since we need to install grub on that dataset in order for the VM artifacts generated by the build to boot correctly. In (2), we don't create rpool/grub since this dataset is re-used even for "not-in-place" upgrades. Thus, ignoring rpool/grub, the datasets generated in (1) and (2) will be identical.

Additionally, based on these guidelines, this means we should remove the creation of rpool/crashdump from appliance-build, and move that to the delphix-platform service.

This issue is meant to track the work needed to move this documentation into a proper place; e.g. some README.md file, or Confluence, or wherever will serve future developers best.