mirrorlist: Optimize the setup_continents method.

Profiling before & after:

ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     1    0.000    0.000    0.005    0.005 mirrorlist_server.py:290(setup_continents)
     1    0.001    0.001    0.001    0.001 mirrorlist_server.py:291(setup_continents)
Merge pull request #9 from fedora-infra/feature/setup_continents-faster

mirrorlist: Optimize the setup_continents method.