Add element to modify /etc/apt/sources.list in dib

Add apt-sources element for Ubuntu OS to override the default
/etc/apt/sources.list in cloudimg.

Set DIB_APT_SOURCES with a proper sources.list file to replace
the default one.

If you want to use this element in tripleo project, set NODE_DIST or
EXTRA_ELEMENTS / UNDERCLOUD_DIB_EXTRA_ARGS / OVERCLOUD_DIB_EXTRA_ARGS
to make it take effect at build time and run time.

e.g. before running devtest.sh:
export DIB_APT_SOURCES=/etc/apt/sources.list
export NODE_DIST="ubuntu apt-sources"

Change-Id: I9ce0d03b506c2948b96382e4d6e85f0aff906450
Merge "Add element to modify /etc/apt/sources.list in dib"