Make sure to use only up-to-date mirrors

Make sure to use only up-to-date mirrors

Whitewash the geographically "close" mirrors against the list of
fully synced mirrors - we've had a few instances of getting bad
mirrors which would break the script.

All credit goes to:

https://askubuntu.com/questions/39922/how-do-you-select-the-fastest-mirror-from-the-command-line

... and we should perhaps use the netselect tool as well somewhere down
the line, but this fixes the most urgent problem.

Change-Id: Ia61a2bd78657b28019f57fc1962d410e754e6d2a
Signed-off-by: Stefan K. Berg <stefan.k.berg@ericsson.com>