On a cloud that has inaccessible version discovery documents AND uses
integer project ids, the discovery fallback logic can fail because the
project id parses as a (very large) version.
Check to see that the url segment in the fallback code begins with a v,
so that we're only attempting to parse versions from actual candidate
segments.
Closes-Bug: #1806109
Change-Id: Id90b3b9e4852494a4678b0a9bb67362babdc971c