Use add-apt-repository for Ubuntu Cloud Archive

add-apt-repository now supports the cloud-archve: prefix; update
instructions for Ubuntu to detail how todo this.

bug 1242483
Cherry-Pick: https://review.openstack.org/#/c/52848/

Change-Id: I2e9c20193a54ecc0ef85e6093bcc7e4ebbf23116
Merge "Use add-apt-repository for Ubuntu Cloud Archive" into stable/havana