Update devstack docs, require Ubuntu 14.04

We no longer test deploying Devstack + Ironic on Precise 12.04 anywhere,
so we should no longer promise that it is functional there.  This removes steps
of the DevStack documentation that sets up access to the Cloud Archive and
explains that Ubuntu 14.04 is the minimum supported OS.

Closes-bug: 1371300

Change-Id: I99b7705c2c78443c7a95ff499b8c9fde33964c46
Merge "Update devstack docs, require Ubuntu 14.04"