Reset @display when viewing drift history screen.

- Added change to reset @display when viewwing drift history screen to fix an issue where @display was still being set to vms/storages etc after clicking on links to relationships from host summary screen and then clicking on Drift History link from listnav on left side it was trying to render incorrect partial due to @display being set incorrectly. Drift History link from right side of the summary screen was working fine because going to summary screen resets @display back to "main"

- Added spec test to make sure @display gets reset by drift_history method.

https://bugzilla.redhat.com/show_bug.cgi?id=1123144
https://bugzilla.redhat.com/show_bug.cgi?id=1126085
Merge pull request #329 from h-kataria/reset_display_instance_variable_when_viewing_drift_history

Reset @display when viewing drift history screen.