[POLICY-54] Add proxy for docker builds and fixes

[POLICY-54] Add proxy for docker builds and fixes

This patch helps to build Policy docker images behind a proxy and
fixes the following correction in patch
https://gerrit.onap.org/r/#/c/5585/ :

1. $HTTP_PROXY and $HTTPS_PROXY environment variables
   gets reset with HTTP_PROXY="" and HTTP_PROXY="" overriding it.

2. When both $HTTP_PROXY and $HTTPS_PROXY are set, a space is needed
   when they are appended for providing correct arguments to docker
   build command.

Change-Id: I1d0f2003c77444e372b754d42f20d2e428e70a95
Signed-off-by: Shashank Kumar Shankar <shashank.kumar.shankar@intel.com>