Simplify the test config format

The current test config format is a kind of legacy from the previous versions
where it had to contain a little bit more information about the tests to be
run. Now there is an opportunity to simplify its format by getting rid of the
'tests_to_run' attribute. Thus the following sample config:

"verify": {
  "tests_to_run": ["sanity", "smoke"]
},
"benchmark": {
  "tests_to_run": {
    "NovaServers.boot_and_delete_server": [
        {"args": {...}, "times": 1, "concurrent": 1}
    ]
  }
}

- now gets simplified to:

"verify": ["sanity", "smoke"],
"benchmark": {
  "NovaServers.boot_and_delete_server": [
      {"args": {...}, "times": 1, "concurrent": 1}
  ]
}

The patch also simplifies the structure of the rally.benchmark.config module,
by getting rid of the redundant TestConfigManager class, as well as removing
the abstract ConfigManager base class.

Change-Id: I1e6b3c7a5798a7e503040c64f4e74805a72cc885
Blueprint test-engine