Bug 1858: Adding APT proxy

Adds apt proxy to build scripts and dockerfiles.  Backward compatible
change and proxy will not be used unless environment variable
is supplied to docker build command.

Fixes bug 1858

Change-Id: Ibd25bd1c145b8d43eeb85c5c952aec30ebcce1fc
Signed-off-by: beierlm <mark.beierl@canonical.com>