Fixes #8266: Allow setting the number of Pulp workers to be used.

This also adds logic to set the Pulp workers based on processor count
but to cap the maximum at 8 unless overriden by a user.
There's a [min](https://github.com/puppetlabs/puppetlabs-stdlib/blob/master/lib/puppet/parser/functions/min.rb) function in stdlib, so I think the following should work, assuming you use at least stdlib 2.6.0 (which is very old):
```puppet
$num_workers = min($::processorcount, 8)
```