Fix unit test

The db_backend_package name has changed in puppet-oslo[0], so fix it.

[0]: https://review.openstack.org/#/c/467582/

Change-Id: Ic8b3c060d7b39226763e3ebb0cb28f8ab971fd4f
Closes-Bug: #1693403

spec/classes/vitrage_db_spec.rb

@@ -84,7 +84,7 @@ describe 'vitrage::db' do
       end
 
       it 'install the proper backend package' do
-        is_expected.to contain_package('db_backend_package').with(
+        is_expected.to contain_package('python-pymysql').with(
           :ensure => 'present',
           :name   => 'python-pymysql',
           :tag    => 'openstack'
@@ -99,7 +99,6 @@ describe 'vitrage::db' do
         { :database_connection => 'mysql+pymysql://vitrage:vitrage@localhost/vitrage', }
       end
 
-      it { is_expected.not_to contain_package('db_backend_package') }
     end
   end