Document Fuel upgrade procedure

User Guide
- 0070-introduction.rst -- mentions upgrade and links to doc
- 8000-upgrade.rst -- details about upgrade process
- create-environment/1500-name-distro -- explanation about multiple
  Ubuntu/CentOS options on the drop-down list if one upgraded
  from earlier MOSt release

contents-user.rst -- include user-guide/8000-upgrade.rst

Closes-bug: 1334463

Change-Id: I13f23ad10364506e273a69a2411b217dd858834f