add chef search discovery. Fixes #40

adds a `search_for_nodes()` helper which searches for other servers.
Defaults to role search within the same chef_environment, but allows for
more complex queries. Also defaults to smart interface selection, using
`cloud.local_ipv4` or `ipaddress` attributes as appropriate, but also
allows for an override.

adds a search_discovery recipe which disables multicast discovery and
populates the unicast hosts with the results of `search_for_nodes()`.