Allow user to set the number of api workers

Defaults to ::processorcount
Also reseting puppet 2.6 compat in spec/classes/nova_db_postgresql_spec.rb
so that unit tests pass, it looks like our 2.6 unit test runner is using
puppet 2.7

Change-Id: I521679ce497a6d9f39a88a6e1d16d5a4ca67255a