lxc-alpine: use dl-cdn.a.o as default mirror instead of random one
lxc-alpine: use dl-cdn.a.o as default mirror instead of random one

Some mirrors from the mirrors list are not very reliable and it seems
that no one really wants to use some random mirror as the default
option.

Signed-off-by: Jakub Jirutka <1d3764b91b902f6b45836e2498da81fe35caf6d6@jirutka.cz>

thmo/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,thmo/lxc,GreatFruitOmsk/lxc,hallyn/lxc,aeris/lxc,federicobriata/lxc,LynxChaus/lxc,aeris/lxc,federicobriata/lxc,terceiro/lxc,GreatFruitOmsk/lxc,GreatFruitOmsk/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,federicobriata/lxc,hallyn/lxc,thmo/lxc,caglar10ur/lxc,LynxChaus/lxc,hallyn/lxc,aeris/lxc,terceiro/lxc,terceiro/lxc,caglar10ur/lxc,ss1h2a3tw/lxc