lxc-alpine: automatic repository selection

pick random server from mirror list
use the latest stable release

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
lxc-alpine: option for specifying the release to be installed

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>