Install Guide: reorder Compute steps for AMQP

* remove duplicated instructions for Ubuntu
* group AMQP settings for all distributions in a same step
* move AMQP settings to a more sensible place in the compute
  controller installation section

Change-Id: I9efd4fe93f189ef4c631afb05f07d646eb57c7c4
backport: havana
(cherry picked from commit 3e0274dee123854cc0d1a8dba9ceaaf937d3948a)
Merge "Install Guide: reorder Compute steps for AMQP" into stable/havana