`Mdm::Host` `os` search operato

[MSP-10028](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&selectedIssue=MSP-10028)

The `Mdm::Host` os search operator will split a search query into words
and then take the union of the search across `Mdm::Host#os_flavor`,
`Mdm::Host#os_name`, and `Mdm::Host#os_sp`.