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


I was thinking about doing that, but `IsDeeply()` doesn't act like gocheck checker. I think some work needs to be done there, that's why I decided to create separate package and from now on put there only gocheckers. Also a lot is not clear for me in test/gocheck.go e.g. there is `DeepEquals` defined there but do we use it? The same for `FileEquals`.

Also if you aim to roll it out someday then putting this into separate package (which is basically a separate project) is a first step. So, I propose from now on to put all checkers there now, and also to work on `IsDeeply` to make it compatible with gocheck checkers.