use constants rather than instance variables

this way and inherited class can actually access them.
override flush method to write udev rules...

the only problem is that so far this udev provider is choking because:

    Error: Could not run: undefined method `find' for nil:NilClass
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/provider.rb:295:in `default_match'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/provider.rb:287:in `default?'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1654:in `block in defaultprovider'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1654:in `each'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1654:in `find_all'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1654:in `defaultprovider'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1159:in `providers_by_source'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/type.rb:1128:in `instances'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/indirector/resource/ral.rb:25:in `search'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/indirector/indirection.rb:267:in `search'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application/resource.rb:226:in `find_or_save_resources'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application/resource.rb:143:in `main'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application.rb:379:in `run_command'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application.rb:371:in `block (2 levels) in run'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application.rb:477:in `plugin_hook'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application.rb:371:in `block in run'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/util.rb:479:in `exit_on_fail'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/application.rb:371:in `run'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/util/command_line.rb:137:in `run'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/lib/puppet/util/command_line.rb:91:in `execute'
    /home/igalic/.gem/ruby/2.1.2/gems/puppet-3.6.2/bin/puppet:8:in `<top (required)>'
    /home/igalic/.gem/ruby/2.1.2/bin/puppet:23:in `load'
    /home/igalic/.gem/ruby/2.1.2/bin/puppet:23:in `<main>'