PCT-808 system info

[PCT-808](https://jira.percona.com/browse/PCT-808) System info:
- [PCT-809](https://jira.percona.com/browse/PCT-809) Pt-summary back-end
- [PCT-810](https://jira.percona.com/browse/PCT-810) PT-MySQL-summary back-end

TODO:
- [x] boxes for tools-ci.percona.com doesn't have installer pt-toolkit
- [x] test real agent with Pablo's front-end


Let's keep the directory structure as flat as possible: I think this can be put in the existing `test/gocheck.go`?  That file should someday be rolled into it's own project because its `IsDeeply()` is quite good.