Allow multiple Unity backend

The current implementation causes duplicated resources when multiple
Unity backends exist.

Change-Id: Ia364951d989a78d05327fd2f2bc64173426382fa
(cherry picked from commit d00ac4804b)

manifests/backend/dellemc_unity.pp

@@ -141,8 +141,8 @@ define manila::backend::dellemc_unity (
   })
 
   # Python library storops is required to run Unity driver.
-  package{'storops':
+  ensure_packages( 'storops', {
     ensure   => $package_ensure,
     provider => 'pip',
-  }
+  })
 }