use modal dialog pattern instead of popover

[gregsheremeta]

Destructive operations such as shutdown and stop VM should present the user with modal confirmation dialogs. The pattern is under active design by the patternfly team, but we can see what it looks like already: patternfly/patternfly-design#632

So instead of this:

We need this:

[gregsheremeta]

cc @jelkosz @sjd78

[mareklibra]

Thanks for reporting it. So far, we wanted to avoid modal dialogs as possible.
Anyway, time changed and if this pattern becomes part of patternfly, it makes sense to follow it.

I can imagine there will be patternfly-react implementation for this. If so, we should use it.

[bond95]

@mareklibra Actually we already have confirmation modal dialog, for deleting NIC or Disks, thus dialog for shutdown button not gonna conflict with current UX.

[mareklibra]

Oh, right. So let's reuse it.

The point was, that patternfly suggests to use modal dialog to confirm.

[gregsheremeta]

@lizsurette @jennyhaines

[sjd78]

@gregsheremeta - @bond95 did this work in #654, and it was in release 1.4.2

[bond95]

Already implemented in redesign.