Download image API to bypass vCenter

This patch introduces a new API to download images. This new API
provides a new signature which gives the ability to bypass vCenter
when downloading images.

Change-Id: Id3cf3f03e1d0622a99573bd090b8ea6b397765e0
Add unit test for VC 5.1 web fault handling

This patch adds unit test for web fault raised by vCenter 5.1.

Change-Id: I6879dcb50db919386facd81d3ba5e7b133f75397
Merge "Download image API to bypass vCenter"