"DROPDOWN_MAX_ITEMS" horizon parameter was hardcoded in a ruby
template. Hence, it was not possible to manage it using heat.
This fix will allow user to manage "DROPDOWN_MAX_ITEMS" from
heat template.
Change-Id: I121a4ad2070cfef173c572a3d25788abbbf16989
Closes-Bug: #1813786
(cherry picked from commit 004142d577)