cephadm: Split multicast interface and unicast_ip in keepalived.conf

cephadm: Split multicast interface and unicast_ip in keepalived.conf

When deploying keepalived, cephadm set interface and unicast_src_ip
in keepalived.conf. However, having both options are set, but
if unicast_src_ip is not in the interface set by 'interface',
the instrances of keepalived will not properly commuicate.

This commit makes both options exclusive, and add an option to set
either one or the other. Default is set to 'interface', as it seems
multicast is the default way to deploy keepalived.

Signed-off-by: Luis Domingues <domingues.luis@protonmail.ch>