ui: increase z-index for install-wizard step (#3414)

Increase z-index for install wizard step to make buttons clickable again.
After the latest refactoring of the styles there was a slight mistake which caused that the buttons for the initial install wizard in the beginning weren't clickable anymore, because they got overlaid by another box.

Author: Philipp Bankonier

ui/css/cloudstack3.css

@@ -194,7 +194,7 @@ body.install-wizard {
 
 .install-wizard .step {
   position: relative;
-  z-index: $z-index-install-wizard2;
+  z-index: $z-index-install-wizard3;
 }
 
 .install-wizard .step .tooltip-info {