Reset value of index if chart is not available.

- Added line of code to reset value of index in @perf_options to fix an issue([] for nil:NilClass error), if user has zoomed in on one of the charts and then changed options such as group by from the options box on the screen and the returning result didn't include the chart user was zoomed in on. With this change user will be presented with all the zoomed out available charts.
- Moved setting of chart_layout variable outside of the if block as perf_gen_chart_layout method might set @perf_options[:index] to nil in case the zoomed in chart is not present for selected options on screen.

https://bugzilla.redhat.com/show_bug.cgi?id=1144622
https://bugzilla.redhat.com/show_bug.cgi?id=1145741
Merge pull request #680 from h-kataria/fix_nil_error_when_changing_group_after_zooming_chart_in

Reset value of index if chart is not available.