Foreman » Installer

Fix nil provider assignment in foreman_config_entry.

Provider `cli` for `foreman_config_entry` can pass nil instead of a
valid provider during prefetch if a config_entry is not found.

This breaks puppet during prefetch:

Error: Could not prefetch foreman_config_entry provider 'cli': undefined
  method `intern' for nil:NilClass
  /opt/ruby2.0/lib/vendor_ruby/2.0.0/puppet/type.rb:1715:in `provider'
  /opt/ruby2.0/lib/vendor_ruby/2.0.0/puppet/type.rb:1928:in `provider='
  /var/lib/puppet/lib/puppet/provider/foreman_config_entry/cli.rb:59:in
  `block in prefetch'
  /var/lib/puppet/lib/puppet/provider/foreman_config_entry/cli.rb:53:in
  `each'
  /var/lib/puppet/lib/puppet/provider/foreman_config_entry/cli.rb:53:in
  `prefetch'
  /opt/ruby2.0/lib/vendor_ruby/2.0.0/puppet/transaction.rb:307:in
  `prefetch'

This commit, correctly, skips provider assignment for config entries that
don't exist.

Closes GH-420