Initial commit/skeleton for "use mock chassis" story

OK - so I got the  "I would like to use ironic mock chassis" story. There's an obvious overlap with the stuff that Jay is doing for "As a user I would like to use mock ironic models" - this is just some basic skeleton for now:
                                                                           
* Chassis model  /model/Ironic/chassis.py (just attributes from [api](https://github.com/openstack/ironic/blob/master/doc/source/dev/api-spec-v1.rst#chassis) )
* Ironic driver chassis methods - tuskar/drivers/ironic_chassis_methods.rb 
  As a user I would like to use ironic mock chassis:                       
                                                                              
  --> get_all_chassis():                                                   
  --> get_chassis(c_id):                                                   
  --> create_chasis(attrs={}):                                             
  --> update_chasis(attrs={}): #e.g. add hosts/nodes to the chassis or remove
  --> delete_chasis(c_id)                                                  
                                                                              
* ironic_common.py is probably going to get overwritten by Jay - meant to hold the util methods like 'store'/'retrieve' for the mock objects

Also an overlap with the Rack - obvious ones:
1) user creates a new Rack. All of the hosts in that rack will be placed into a newly created Ironic chassis.
2) user adds a host to a Rack. The chassis corresponding to the rack should be updated to include the new host.
3) user lists a Rack - the chassis attribute is returned in the serialization.
4) user lists all Racks - chassis attribute on all Rack.                   
5) user deletes a Rack - delete the associated chassis.                    




                                                                           
marios