The "ubuntu" target had a post-install 99-autoremove task that removed
unnecessary dependency packages, but the "ubuntu-minimal" target does not.
This patch moves the 99-autoremove post-install task from the "ubuntu" target
to the "ubuntu-common" target so that both will run an autoremove at the end of
the image build.
For the Octavia amphora image, this saved about 1GB in the image by removing
build only package dependencies.
Closes-Bug: #2012406
Change-Id: I4592e3bd502045fa89203c075d3ea8f632e77177
