HRoller design updates

- Specify that there will be options for selecting nodes by at least
  nodegroups and tags, rather than just individually.
- Specify a better handling for non-redundant instances (eg. plain or
  file) which today are simply ignored
- Specify that the rolling maintenance behavior is triggered by
  instances being up, but also overridable
- Remove execution of rolling maintenances altogether, as it is deemed
  unsafe in the current version, and move it to future work, discuss the
  requirements that were pointed out for it to be safe.

Cosmetic:
- Fix numbered list, which were rendered incorrectly in the HTML version

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>