requests.Session brings the following benefits which
are not easily available by invoking requests.request():
* Connection pooling
* Common place for configuring all requests
* Configurable auth mechanism for the future use
This patch also introduces HTTP_TIMEOUT and HTTP_PROXY
configuration options which are used to set the appropriate
requests' parameters
Closes-bug: #1459964
Change-Id: Iaee70768a3790a7a25c047aba2d19a058facda21