Support general configuration file

Support general configuration file

Use openstack library oslo_config for parsing configuration options
from the command line and configuration files.
Refer http://docs.openstack.org/developer/oslo.config/ or rally
source code for more info on oslo_config library usage.

This patch is initially for test result dispatcher configuration,
but it is very general to use.

Usage:
  0) install yardstick
  1) mkdir /etc/yardstick
  2) cp <repo_root_dir>/etc/yardstick/yardstick.conf.sample \
        /etc/yardstick/yardstick.conf
  3) edit /etc/yardstick/yardstick.conf
  4) run `yardstick task start xxx` cmd

JIRA: YARDSTICK-61

Change-Id: I01677ef6e9ab7c1975aa193799195e850da73478
Signed-off-by: QiLiang <liangqi1@huawei.com>