build: Add local Ubuntu mirror to target image

build: Add local Ubuntu mirror to target image

When deploying via Jenkins CI, the repos used for target image
building are overridden to use the local Ubuntu mirror on Fuel
Master instead of fetching packages from internet.

However, when deploying manually, only bootstrap image building
benefits from the local Ubuntu mirror.

While keeping the internet mirrors, add the local Ubuntu mirror
to target image build repo list.

JIRA: ARMBAND-35

Change-Id: Ibd58067703ce6370781a6b54fd01ce52e29686ac
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>