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


This confused me at first.  I think the more common Go style is:

```go
service, ok := m.service[serviceName]
if !ok {
   // error
}
service.Whatever()
```