fix #2773: ebtables: keep policy of custom chains

fix #2773: ebtables: keep policy of custom chains

currently all ebtalbes chains are created with a hardcoded policy of ACCEPT.
This patch changes the functionality to store the configured policy of a
chain while reading the 'ebtables-save' output and uses this policy when
creating the command list.

This is only relevant for ebtablers chains not generated by pve-firewall (the
ones having an action of 'ignore' in the status-hash).

Reported on the pve-user list:
https://pve.proxmox.com/pipermail/pve-user/2020-May/171731.html

Minimally tested with the example from the thread.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>