Use custom transport adapter for file URLs

The requests library doesn't support local file URLs. The PBM WSDL is
stored locally and this affects all PBM API invocations. This patch
fixes the problem by registering a custom transport adapter for local
files.

Closes-Bug: #1371575
Change-Id: Ie0635607847061c68c879e87db8f2541436f03d6
Merge "Use custom transport adapter for file URLs"