In other places in the codebase, we cache a negative result for less
time (usually a tenth) than we cache a positive result. This commit
makes TempURL do the same.
This creates a slightly nicer experience for someone trying to set up
TempURL; if they get the initial key setup wrong, try it, then fix it,
it now takes only 6 seconds until they can use signed URLs, not
60. That's a short enough time that they may not even notice the
caching of no keys.
Change-Id: I521f023e7cddaecd07f0ce32aedd4059bd0b8ec4
2c9aee55fb