Improve first time download of ubuntu images.

If a cached copy of the file doesn't exist, cache_url() passes a
non-existent path to -z/--time-cond and you see this warning:

 Warning: Illegal date format for -z, --timecond (and not a file name).
 Warning: Disabling time condition. See curl_getdate(3) for valid date syntax.

It works just fine, but the warning is ugly.

Change-Id: Ic6f13a2c596b988308d7fca9cd1745e5d48ae5fb
Merge "Improve first time download of ubuntu images."