Update apt cache before installing UCA keyring

add-apt-repository will attempt to install the ubuntu-cloud-keyring package.
Call 'apt-get update' first to ensure the local cache is up to date and the
package can be found.

This also updates dsg to only set this up if running on Precise.

Change-Id: Ie349e89ef20ac4c526c8353cf4aabe3ddad73824
Partial-bug: #1326754.
Merge "Update apt cache before installing UCA keyring"