keepalived: avoid empty auth config when disabled

When disabling VRRP AUTH, avoid having an empty configuration in
keepalived.conf like this:

  authentication {
      auth_type
      auth_pass
  }

And have errors when starting Keepalived.
Setting to False by default ensure to not have this situation.

Also, ensure we use an array for keepalived_internal_ipvs parameter to
make the difference possible even if the param is not an array.
Merge pull request #691 from enovance/keepalived-auth

keepalived: avoid empty auth config when disabled