corosync: allow to set link priorities

corosync: allow to set link priorities

For now in passive mode, a link with a higher value has a lower
priority. If the current active link fails the one with the next
higher priority will take over. Use 255 as maximum, as internally
kronosnet uses an uin8_t variable for this, and while there can be
"only" 8 links currently it may be still nice to use different values
that ]0..1[ for them, e.g., when re-shuffling link priorities it's
useful to have space between them.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>