Add support for DNF

If the folder `/car/cache/dnf` is present, the content will be cached
like in the Yum plugin.

A possible configuration to enable it:

Vagrant.configure(2) do |config|
   config.vm.box = "fedora/24-cloud-base"
  if Vagrant.has_plugin?("vagrant-cachier")
    config.cache.scope = :machine
    config.cache.auto_detect = false
    config.cache.enable :dnf
  end
end

docs/buckets/dnf.md

@@ -0,0 +1,20 @@
+# DNF
+
+Used by Fedora guests, will get configured under guest's `/var/cache/dnf`. It will
+also [make sure](lib/vagrant-cachier/bucket/dnf.rb#L20) that `keepcache` is set to
+`1` on guest's `/etc/dnf/dnf.conf`.
+
+To manually enable it:
+
+```ruby
+Vagrant.configure("2") do |config|
+  config.vm.box = 'some-fedora-box'
+  config.cache.enable :dnf
+end
+```
+
+### :warning: Notice about Windows hosts :warning:
+
+In case this bucket is enabled and a Windows host is in use, you might see an
+ugly stacktrace as described on [this comment](https://github.com/fgrehm/vagrant-cachier/issues/117#issuecomment-50548393)
+if some DNF repository is not available during provisioning.

docs/template.html

@@ -89,6 +89,7 @@
                   <li><a tabindex="-1" href="buckets/chef">Chef</a></li>
                   <li><a tabindex="-1" href="buckets/chef_rubygems">Chef Gems</a></li>
                   <li><a tabindex="-1" href="buckets/composer">Composer</a></li>
+                  <li><a tabindex="-1" href="buckets/dnf">DNF</a></li>
                   <li><a tabindex="-1" href="buckets/bower">Bower</a></li>
                   <li><a tabindex="-1" href="buckets/pacman">Pacman</a></li>
                   <li><a tabindex="-1" href="buckets/npm">npm</a></li>

lib/vagrant-cachier/bucket.rb

@@ -93,6 +93,7 @@ def symlink?(path)
 require_relative "bucket/chef_gem"
 require_relative "bucket/pacman"
 require_relative "bucket/yum"
+require_relative "bucket/dnf"
 require_relative "bucket/rvm"
 require_relative "bucket/apt_cacher"
 require_relative "bucket/apt_lists"

lib/vagrant-cachier/bucket/dnf.rb

@@ -0,0 +1,26 @@
+module VagrantPlugins
+  module Cachier
+    class Bucket
+      class Dnf < Bucket
+        def self.capability
+          :dnf_cache_dir
+        end
+
+        def install
+          if guest.capability?(:dnf_cache_dir)
+            guest_path = guest.capability(:dnf_cache_dir)
+            return if @env[:cache_dirs].include?(guest_path)
+
+            # Ensure caching is enabled
+            comm.sudo("sed -i '/keepcache=/d' /etc/dnf/dnf.conf")
+            comm.sudo("sed -i '/^[main]/a keepcache=1' /etc/dnf/dnf.conf")
+
+            symlink(guest_path)
+          else
+            @env[:ui].info I18n.t('vagrant_cachier.skipping_bucket', bucket: 'DNF')
+          end
+        end
+      end
+    end
+  end
+end

lib/vagrant-cachier/cap/redhat/dnf_cache_dir.rb

@@ -0,0 +1,13 @@
+module VagrantPlugins
+  module Cachier
+    module Cap
+      module RedHat
+        module DnfCacheDir
+          def self.dnf_cache_dir(machine)
+            '/var/cache/dnf'
+          end
+        end
+      end
+    end
+  end
+end

lib/vagrant-cachier/capabilities.rb

@@ -61,6 +61,11 @@ class Plugin < Vagrant.plugin('2')
         Cap::RedHat::YumCacheDir
       end
 
+      guest_capability 'redhat', 'dnf_cache_dir' do
+        require_relative 'cap/redhat/dnf_cache_dir'
+        Cap::RedHat::DnfCacheDir
+      end
+
       guest_capability 'suse', 'yum_cache_dir' do
         # Disable Yum on suse guests
       end