Modify the CLI output for 'task detailed'

1. Join the 2 output tables (atomic actions & overall results) into one table.
   Format the float numbers to make the table more neat.

Before: http://paste.openstack.org/show/76847/
After:  http://paste.openstack.org/show/77630/

2. Do not use the PrettyTable lib directly, but use openstack.common.cliutils.

3. Remove the --no-aggregation parameter that previously allowed to switch
   between 2 modes of atomic action tables. Always show the 'default' atomic
   actions table and inroduce a new --iterations-data parameter, which makes
   Rally show the detailed information about iteration durations in a separate
   table below the main one.

Change-Id: I942b6d461e1e649f12ef0085ddd86677e8cbcee2