debian: install cloud-init on Wheezy

cloud-init packages are not available in the Debian Wheezy repository.
However, they are available in wheezy-backports.

With this change, the cloud-init packages are installed from
wheezy-backports if DIB_RELEASE is wheezy.

Change-Id: I3daa3b0ba185ba3f7c1bf6b8f9b1f8c167fcb340
Merge "debian: install cloud-init on Wheezy"