Nova client for Flavor model

Hi                                                                         
                                                                           
this adds code for creating and deleting flavors in nova, using python-novaclient.
                                                                           
* added python-novaclient and simpleyaml to requirements                   
* uses yaml config file for nova @ etc/tuskar/nova_overcloud_config.yml    
* Adds a nova_flavor_uuid attribute to Flavor db model (not exposed - used to track nova flavors)
* Added calls to create/delete nova Flavors inside the Tuskar controllers  
                                                                           
thanks, marios