5.1.1 Upgrade/update instructions

user-guide -- 8000-upgrade.rst

- rewrite information describing upgrade/update capabilities for 5.1.1
- add password info (other information about new authx features is in a
  separate commit

ref-arch -- 8000-fuel-upgrade -- changed release numbers

Change-Id: I9ae4e69e8c8012b9f41f12dfd940155903f7fcf5
Merge "5.1.1 Upgrade/update instructions" into stable/5.1