lxc-alpine: autodetect standard bridges and set hwaddress

Check for lxcbr0, virbr0 and br0 and use one of those if they exist.

Set mac address if network type is veth.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>