Simplify and improve "rally info"

1. Typing "rally info list" now results in a list of entities available in Rally:

    $ rally info list
    Benchmark scenario groups:
    ----------------------------------------------------------------------------------
     Name                       Description
    ----------------------------------------------------------------------------------
     Authenticate               This class should contain authentication mechanism.
     CeilometerAlarms           ...

    ...

   The same info appears on just "rally info" (we've monkey-patched InfoCommands
   docstring to achieve this).

2. "rally info find" called for benchmark scenario groups now prints a list of benchmark
   scenarios available there with short descriptions.

    $ rally info find NovaServers
    NovaServers (benchmark scenario group).

    Benchmark scenarios:
    --------------------------------------------------------------------------------
     Name                                             Description
    --------------------------------------------------------------------------------
     NovaServers.boot_and_bounce_server               Test booting a server (...)
     NovaServers.boot_and_delete_server               Tests booting and then (...)
     NovaServers.boot_and_list_server                 Tests booting an image (...)

3. Minor local code & UTs improvements

Change-Id: Ib954796245d12ee0aed793984233c3d36ad24ad5
Merge "Simplify and improve "rally info""