Support for version and extension discovery
    
    - Supports unauthenticated call to Keystone to discover
      supported API versions
    - Added command-line support (usage: keystone discover)
    - Added client support (keystoneclient.genenric client).
      Client returns dicts, whereas shell command prints
      formated output.
    - Added tests for genenric client
    - Replicates 'nove discover' in python-novaclient
    - Starts to address blueprint keystone-client
    - keystone discover output looks like this:
        $ keystone discover
        Keystone found at http://localhost:35357
            - supports version v1.0 (DEPRECATED) here http://localhost:35357/v1.0
            - supports version v1.1 (CURRENT) here http://localhost:35357/v1.1
            - supports version v2.0 (BETA) here http://localhost:35357/v2.0
                - and HP-IDM: HP Token Validation Extension
                - and OS-KSADM: Openstack Keystone Admin
                - and OS-KSCATALOG: Openstack Keystone Catalog
    
    Change-Id: Id16d34dac094c780d36afb3e31c98c318b6071ac