refactor client.pp to support a rhn satellite 

this pull request adds rhn satelllite support to spacewalk::client. i've introduced two new parameters:

- use_spacewalk_client_repo: should spacewalk::client use the client repo. when using a rhn 
  satellite the spacewalk client repo must not be used, as redhat already includes the required rpm's
  already in the base channel.

  Default is yes, so the behaviour of the class does not change

- force_registration: this re-registers the client to the spacewalk server even if the systemid file exists.
  thats useful if the spacewalk server changes, but currently has to be used with great care as it may
  result in the same client registered more than once to the server.

i've also included some rspec tests for client .pp and updated the documentation.

thanks
toni
  Default is false.