Routing table acceptance tests were added and some small changes

  * function for creating `elliptics.Session` can be invoked to logging in
  specific file
  (it allow each test to have separate logging file)
  * logging level for `consoleHandler` was changed to **ERROR**
  (`consoleHandler` is a default handler, which can be used (with inheritance,
  for example) by anyone, and it is bad idea to set it's logging level to change
  your own handler's (which inherit `consoleHandler`) logging level)
  * `drop`/`resume_node` functions were added to `test_helper.network` module
  (`EllipticsTestHelper.drop_node` and `EllipticsTestHelper.resume_node` are
  depricated and will be removed)
  * `ssh` module was added to `test_helper` package
  (it is a bad practice to use shell commands through python, and all new tests
  should use this module to execute SSH commands on remote hosts; this module
  uses `paramiko` - python SSH module)
  * `nodes` fixture was added to global `conftest.py` file
  (`nodes` fixture from `test_helper.elliptics_testhelper` is depricated;
  importing fixtures is a bad idea, when it can be placed in `conftest.py` file)

**reviewer**: @uvizhe
Также, не вижу смысла отдавать пользователю дальнейшую работу с методами paramiko (exec_command) и обертками над ними (ssh.wait_for_command). Если уж ты делаешь удобный инструмент для работы с ssh, то почему не сократить число вызовов до одного? ssh.exec(cmd). и пусть она внутри делает все эти exec_command и wait_for_command.