use reactive programming for "mock init" and dnf loading

[jsilhan]

DNF should be initialized within Mock installroot directory. The task will be to:

• investigate whether we can use https://github.com/ReactiveX/RxPY library
• check if check_output is compatible with RxPY
• chain Command("mock init ...").execute() and dnf loading within installroot directory
• let dnf_sack be Observable and treat it like that via RPG and it's plugins

This issue should fix loading custom repositories of the host when dnf sack is initialized. QtSchleduler could be used for GUI integration afterwards.

[jsilhan]

@cibo94 , any progress on this? (It's should be actually #255)