Show track size metrics when listing tracks

[danielmitterdorfer]

As requested in elastic/rally-tracks#29, we should calculate the number of documents and the size of the track files for each track and show it when listing tracks. Example:

daniel@io:~ $ esrally list tracks

    ____        ____
   / __ \____ _/ / /_  __
  / /_/ / __ `/ / / / / /
 / _, _/ /_/ / / / /_/ /
/_/ |_|\__,_/_/_/\__, /
                /____/

[INFO] Writing logs to /Users/daniel/.rally/logs/rally_out_20170629T081838Z.log
Available tracks:

   Name        Description                                                                   Documents  Compressed Size    Uncompressed Size    Default Challenge        All Challenges
   ----------  --------------------------------------------------------------------------  -----------  -----------------  -------------------  -----------------------  --------------------------
   geonames    Standard track in Rally (11.4M POIs from Geonames)                             11396505  252.4 MB           3.3 GB               append-no-conflicts      append-no-conflicts,app...
   geopoint    60.8M POIs from PlanetOSM                                                      60844404  481.9 MB           2.3 GB               append-no-conflicts      append-no-conflicts,app...
   logging     Logging benchmark                                                             247249096  1.2 GB             31.1 GB              append-no-conflicts      append-no-conflicts,app...
   nested      Nested query benchmark using up to 11,203,029 questions from StackOverflow     11203029  663.1 MB           3.4 GB               nested-search-challenge  nested-search-challenge...
   nyc_taxis   Trip records completed in yellow and green taxis in New York in 2015          165346692  4.5 GB             74.3 GB              append-no-conflicts      append-no-conflicts,app...
   percolator  Percolator benchmark based on 2M AOL queries                                    2000000  102.7 kB           104.9 MB             append-no-conflicts      append-no-conflicts,app...
   pmc         Full text benchmark containing 574.199 papers from PMC                           574199  5.5 GB             21.7 GB              append-no-conflicts      append-no-conflicts,app...

-------------------------------
[INFO] SUCCESS (took 2 seconds)
-------------------------------

[danielmitterdorfer]

We've amended the output to show also compressed and uncompressed size of the track.