HTTPS Proxy support for A1 adapter southbound

Provision to configure Http Proxy server information in A1 Adapter so that the southbound calls will pass through proxy server.

There are 2 ways to do this,

1. Update the SLI plugin to configure the http proxy server details so that any A1 request use proxy call.
2. Update the DG so that it allows the request to use the http proxy server which is already configured in properties.

Option 1 will require regression test as lot of other components in SDNC/SDNR uses this plugin.