Add System search and filtering using inventory date/status.

This patch adds support for two things:

1. Searching systems via the Inventoried status or date from the CLI.

2. Recipes can also specify an ``last_inventoried`` element to ask Beaker
   to schedule it on a system inventoried on/after/before a certain
   date or whether inventoried at all.

Change-Id: If819f8efafb15d8c92aca21ba2171216137ca0ca
Bug: 949777