Corrects doc to build appliance using jessie release
Corrects doc to build appliance using jessie release

Currect docs image build instructions use the wheeze release
which will should be the jessie release.

Change-Id: I3a1d65dd68c4a8d24731b6c93576f72248a3310f
Closes-Bug: 1531971

openstack/akanda-rug,stackforge/akanda-rug,stackforge/akanda-rug,openstack/akanda-rug