Add functions to resolve hostnames and iterate corosync nodes

Add functions to resolve hostnames and iterate corosync nodes

The sub 'for_all_corosync_addresses' iterates through all nodes in a
passed corosync config and calls a specified function for every ringX_addr
on every node it finds (provided the IP-version matches the specified
one or undef was specified).

All ringX_addr entries that cannot be parsed as an IP address will be
best-effort resolved as hostnames. This has to happen in the exact same
way as corosync does internally, to ensure consistency with firewall
rules.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>