In one of the places where endpoint_override is used we did not strip
the /v1 suffix, resulting in doube /v1/v1 sometimes.
This patch also corrects the way we strip this suffix (str.rstrip accepts
a set of symbols, not a substring; use regex instead).
It remains unclear why the breakage passed the gate initially. I assume
it was not on the active code path until some Nova change.
Story: #2005723
Task: #31051
Change-Id: I3b25f4fb170aa93159ffa8074dc74fa6f50671b7
4565af8ccd