Hroller design doc

Hroller in 2.7 was implemented (as an experiment) but never explicitly
designed. This design mentions the current functionality, and explains
the changes that will happen for the next release to support live
maintenance scheduling (rolling reboot).

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