Add unicast support

This pull-request adds unicast support to the keepalived puppet module. The only caveat is that you need a recent version of keepalived. I think it is useful for people who cannot use the default multicast mechanism. I've tested this with the latest version of keepalived (1.2.13). I don't know how to check the keepalived version during the puppet run to maybe issue a warning when using the unicast settings. Maybe the version could be made explicit in the module, just a suggestion.