Add kolide_softwareupdate_scan table

[RebeccaMahany]

We are occasionally seeing the new kolide_softwareupdate table return 0 rows from devices who should have a software update available. The explanation appears to be that those devices have not scanned for updates recently -- so this PR adds a second table kolide_softwareupdate_scan that will perform the scan. With this data, hopefully we should be able to determine whether the lack of scan is why kolide_softwareupdate sometimes returns 0 rows when it should return some.

[directionless]

I think the right way is to plumb some options through the exec helper. But this seems like a fine hack