Pick up proxy buildargs from environment

We currently support any buildarg via --build-args. This patch picks up
the default supported proxy buildargs if set in the user's environment,
to allow for more transparent proxy support.

The --build-args will take precendence.

DocImpact
Implements blueprint: better-proxy-support

Change-Id: I084e5d1cc8f9993d65167878f9983ad58a68e86e