Add rsync to the package list used by debootstrap

Debian builds fail due to missing rsync in the baseline set
of packages that debootstrap defaults to. Rsync is used by a
variety of tripleo-image-elements and should be included in
disk images by default.

Change-Id: I8d83a53dfab279075e11f9825f15ee960395dd17
Merge "Add rsync to the package list used by debootstrap"